Forums

My first flyback, Help!

Started by George Herold April 18, 2018
Hi all, well I went and moded JL's flyback with the LTC3803. 
I stuck the file in dropbox, but if someone can't access that I'll
be happy to copy it here.  
https://www.dropbox.com/s/gd4zngedu9za4dc/GH_flyback_2.asc?dl=0 

For the inductor I picked one of the Versa-Pak inductors with 6 coils 
on one form.  And made a 1:5 transformer.  So I first assumed I'd be 
running in continuous conduction mode.  (Which may be wrong.) And went 
ahead and calculated my duty cycle ratio.  
V_out(300)/V_in(24) = n*D_on/D_off.. or it's on ~30% of the time.  
and then the 'on' current from the 24V.  ~150mA*   So I picked 
an inductor with a saturation current which is twice this. 
(I have no idea how to pick the inductor.)  Well that didn't work!
I had to increase the current a lot to get the output up to 300V.

I needed closer to 1 amp to get the thing to work.  So A different 
inductor.  (VPH5-0155-R)  

I must admit that if I just look at the inductor.  
V = L*di/dt that at 24V and ~24uH I expect the current to increase 
at about 1 amp/ usec. which is about what I see.  

So any advice about what I did wrong, or how to do this better would 
be appreciated.  
I should also note that there is lots of ringing in the inductor.  
I'm not sure what's causing this.    
    
George H. 


*This is just from 100% energy balance and is different from the equation in 
my SMPS text book.     
George Herold <gherold@teachspin.com> wrote:
> Hi all, well I went and moded JL's flyback with the LTC3803. > I stuck the file in dropbox, but if someone can't access that I'll > be happy to copy it here. > https://www.dropbox.com/s/gd4zngedu9za4dc/GH_flyback_2.asc?dl=0 > > For the inductor I picked one of the Versa-Pak inductors with 6 coils > on one form. And made a 1:5 transformer. So I first assumed I'd be > running in continuous conduction mode. (Which may be wrong.) And went > ahead and calculated my duty cycle ratio. > V_out(300)/V_in(24) = n*D_on/D_off.. or it's on ~30% of the time. > and then the 'on' current from the 24V. ~150mA* So I picked > an inductor with a saturation current which is twice this. > (I have no idea how to pick the inductor.) Well that didn't work! > I had to increase the current a lot to get the output up to 300V. > > I needed closer to 1 amp to get the thing to work. So A different > inductor. (VPH5-0155-R) > > I must admit that if I just look at the inductor. > V = L*di/dt that at 24V and ~24uH I expect the current to increase > at about 1 amp/ usec. which is about what I see. > > So any advice about what I did wrong, or how to do this better would > be appreciated. > I should also note that there is lots of ringing in the inductor. > I'm not sure what's causing this. > > George H. > > > *This is just from 100% energy balance and is different from the equation in > my SMPS text book.
Your dropbox link works for me. But when your circuit is simulated on my ancient version of LTSpice IV, it displays an output voltage of +21VDC. 73, -- Don Kuenz, KB7RPU Genius is nothing but a greater aptitude for patience. - de Buffon
On Wednesday, April 18, 2018 at 1:00:56 PM UTC-4, nor...@googlegroups.com wrote:
> George Herold <gherold@teachspin.com> wrote: > > Hi all, well I went and moded JL's flyback with the LTC3803. > > I stuck the file in dropbox, but if someone can't access that I'll > > be happy to copy it here. > > https://www.dropbox.com/s/gd4zngedu9za4dc/GH_flyback_2.asc?dl=0 > > > > For the inductor I picked one of the Versa-Pak inductors with 6 coils > > on one form. And made a 1:5 transformer. So I first assumed I'd be > > running in continuous conduction mode. (Which may be wrong.) And went > > ahead and calculated my duty cycle ratio. > > V_out(300)/V_in(24) = n*D_on/D_off.. or it's on ~30% of the time. > > and then the 'on' current from the 24V. ~150mA* So I picked > > an inductor with a saturation current which is twice this. > > (I have no idea how to pick the inductor.) Well that didn't work! > > I had to increase the current a lot to get the output up to 300V. > > > > I needed closer to 1 amp to get the thing to work. So A different > > inductor. (VPH5-0155-R) > > > > I must admit that if I just look at the inductor. > > V = L*di/dt that at 24V and ~24uH I expect the current to increase > > at about 1 amp/ usec. which is about what I see. > > > > So any advice about what I did wrong, or how to do this better would > > be appreciated. > > I should also note that there is lots of ringing in the inductor. > > I'm not sure what's causing this. > > > > George H. > > > > > > *This is just from 100% energy balance and is different from the equation in > > my SMPS text book. > > Your dropbox link works for me. But when your circuit is simulated on > my ancient version of LTSpice IV, it displays an output voltage of > +21VDC. > > 73, > > -- > Don Kuenz, KB7RPU > > Genius is nothing but a greater aptitude for patience. - de Buffon
Hmm well with a 24V supply 21V is pretty wimpy. Does this work any better? I cleaned up the ringing by playing around with an RC across the coil. (a bit of hunt and peck.) Version 4 SHEET 1 2332 704 WIRE -16 -112 -96 -112 WIRE 144 -112 64 -112 WIRE 240 -112 144 -112 WIRE 384 -112 240 -112 WIRE 592 -112 384 -112 WIRE 784 -112 672 -112 WIRE 944 -112 848 -112 WIRE 1088 -112 944 -112 WIRE 1184 -112 1088 -112 WIRE 1264 -112 1184 -112 WIRE 1312 -112 1264 -112 WIRE 384 -80 384 -112 WIRE 592 -64 592 -112 WIRE 672 -64 672 -112 WIRE -96 -48 -96 -112 WIRE 144 -48 144 -112 WIRE 944 -32 944 -112 WIRE 1088 -32 1088 -112 WIRE 1184 -32 1184 -112 WIRE -96 64 -96 32 WIRE -16 64 -96 64 WIRE 144 64 144 32 WIRE 144 64 48 64 WIRE 384 64 384 -16 WIRE 464 64 384 64 WIRE 592 64 592 16 WIRE 592 64 544 64 WIRE 672 64 672 16 WIRE 720 64 672 64 WIRE 752 64 720 64 WIRE 592 96 592 64 WIRE 672 96 592 96 WIRE 1088 96 1088 32 WIRE 1184 96 1184 48 WIRE -96 112 -96 64 WIRE 672 112 672 96 WIRE 144 144 144 64 WIRE 352 192 272 192 WIRE 576 192 432 192 WIRE 624 192 576 192 WIRE 944 240 944 48 WIRE 944 240 800 240 WIRE 16 256 -96 256 WIRE 352 256 272 256 WIRE 480 256 432 256 WIRE 528 256 480 256 WIRE 640 256 608 256 WIRE 672 256 672 208 WIRE 672 256 640 256 WIRE 672 288 672 256 WIRE -96 304 -96 256 WIRE 480 304 480 256 WIRE 944 304 944 240 WIRE 304 320 272 320 WIRE 480 400 480 368 WIRE 672 400 672 368 WIRE -96 416 -96 368 WIRE -16 416 -96 416 WIRE 144 416 144 368 WIRE 144 416 64 416 WIRE 944 432 944 384 WIRE 304 448 304 320 WIRE 576 448 304 448 WIRE 800 448 800 240 WIRE 800 448 576 448 WIRE 144 464 144 416 FLAG 144 464 0 FLAG 672 400 0 FLAG 1088 96 0 FLAG 1184 96 0 FLAG 1264 -112 HV1 FLAG 944 432 0 FLAG 240 -112 IN FLAG -96 112 0 FLAG 720 64 QD FLAG 576 192 QG FLAG 640 256 QS FLAG 576 448 FB FLAG 480 400 0 FLAG 752 64 0 SYMBOL PowerProducts\\LTC3803 144 256 R0 SYMATTR InstName U1 SYMBOL ind2 576 -80 R0 WINDOW 0 -17 36 Right 2 WINDOW 3 -8 71 Right 2 SYMATTR InstName L1 SYMATTR Value 22&micro; SYMATTR Type ind SYMATTR SpiceLine Ipk=1 Rser=0.07 Cpar=1pF SYMBOL ind2 688 32 R180 WINDOW 0 -22 77 Right 2 WINDOW 3 -12 42 Right 2 SYMATTR InstName L2 SYMATTR Value 550&micro; SYMATTR Type ind SYMATTR SpiceLine Ipk=1 Rser=.35 Cpar=2pF SYMBOL nmos 624 112 R0 WINDOW 0 114 48 Left 2 WINDOW 3 75 84 Left 2 SYMATTR InstName Q1 SYMATTR Value Si9420DY SYMBOL res 656 272 R0 WINDOW 0 66 54 Left 2 WINDOW 3 48 88 Left 2 SYMATTR InstName R1 SYMATTR Value 50m SYMBOL res 128 -64 R0 WINDOW 0 -50 33 Left 2 WINDOW 3 -52 64 Left 2 SYMATTR InstName R2 SYMATTR Value 5K SYMBOL res 928 64 M180 WINDOW 0 -69 72 Left 2 WINDOW 3 -90 37 Left 2 SYMATTR InstName R3 SYMATTR Value 3Meg SYMBOL res 928 400 M180 WINDOW 0 -74 64 Left 2 WINDOW 3 -80 31 Left 2 SYMATTR InstName R4 SYMATTR Value 8k SYMBOL cap 1072 -32 R0 WINDOW 0 -61 25 Left 2 WINDOW 3 -64 58 Left 2 SYMATTR InstName C1 SYMATTR Value 66n SYMATTR SpiceLine Rser=10m SYMBOL res 80 400 R90 WINDOW 0 74 86 VBottom 2 WINDOW 3 47 30 VTop 2 SYMATTR InstName R5 SYMATTR Value 20K SYMBOL cap -80 368 R180 WINDOW 0 63 47 Left 2 WINDOW 3 58 14 Left 2 SYMATTR InstName C2 SYMATTR Value 10n SYMBOL voltage -96 -64 R0 WINDOW 123 -220 84 Left 2 WINDOW 0 50 34 Left 2 WINDOW 3 50 66 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 24 SYMBOL res 1168 -48 R0 WINDOW 0 67 38 Left 2 WINDOW 3 64 71 Left 2 SYMATTR InstName R40 SYMATTR Value 100k SYMATTR SpiceLine pwr=12 SYMBOL res -32 -96 R270 WINDOW 0 43 61 VTop 2 WINDOW 3 -9 58 VBottom 2 SYMATTR InstName Rmon SYMATTR Value 1m SYMBOL diode 784 -96 R270 WINDOW 0 -31 30 VTop 2 WINDOW 3 -32 31 VBottom 2 SYMATTR InstName D2 SYMATTR Value HVD SYMBOL res 448 240 R90 WINDOW 0 68 52 VBottom 2 WINDOW 3 75 53 VTop 2 SYMATTR InstName R7 SYMATTR Value 5K SYMBOL res 448 176 R90 WINDOW 0 -44 52 VBottom 2 WINDOW 3 -37 53 VTop 2 SYMATTR InstName R8 SYMATTR Value 33 SYMBOL cap 48 48 R90 WINDOW 0 60 70 VBottom 2 WINDOW 3 32 -2 VTop 2 SYMATTR InstName C11 SYMATTR Value 10&micro; SYMBOL cap 464 304 R0 WINDOW 0 51 26 Left 2 WINDOW 3 52 59 Left 2 SYMATTR InstName C16 SYMATTR Value 1p SYMBOL res 624 240 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R11 SYMATTR Value 100 SYMBOL cap 368 -80 R0 SYMATTR InstName C3 SYMATTR Value 1n SYMBOL res 560 48 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 500 TEXT 632 -160 Bottom 2 !K1 L1 L2 1 TEXT -472 -480 Left 2 !.tran 0 30m 0m startup TEXT 568 -224 Left 2 ;VPH5-0155-R 1:5 TEXT 960 240 Left 2 ;0.8V TEXT 168 -536 Left 2 ;HV SUPPLY J LARKIN Oct 11, 2015 TEXT 192 -488 Left 2 ;Severly modified by G Herold TEXT 1216 -80 Left 2 ;+300 TEXT -504 -432 Left 2 !.model HVD D(Vfwd=0.7) TEXT 288 -352 Left 2 ;Eff ~~ unknown
On Wednesday, April 18, 2018 at 12:16:36 PM UTC-4, George Herold wrote:
> Hi all, well I went and moded JL's flyback with the LTC3803. > I stuck the file in dropbox, but if someone can't access that I'll > be happy to copy it here. > https://www.dropbox.com/s/gd4zngedu9za4dc/GH_flyback_2.asc?dl=0 > > For the inductor I picked one of the Versa-Pak inductors with 6 coils > on one form. And made a 1:5 transformer. So I first assumed I'd be > running in continuous conduction mode. (Which may be wrong.) And went > ahead and calculated my duty cycle ratio. > V_out(300)/V_in(24) = n*D_on/D_off.. or it's on ~30% of the time. > and then the 'on' current from the 24V. ~150mA* So I picked > an inductor with a saturation current which is twice this. > (I have no idea how to pick the inductor.) Well that didn't work! > I had to increase the current a lot to get the output up to 300V. > > I needed closer to 1 amp to get the thing to work. So A different > inductor. (VPH5-0155-R) > > I must admit that if I just look at the inductor. > V = L*di/dt that at 24V and ~24uH I expect the current to increase > at about 1 amp/ usec. which is about what I see. > > So any advice about what I did wrong, or how to do this better would > be appreciated. > I should also note that there is lots of ringing in the inductor. > I'm not sure what's causing this. > > George H. > > > *This is just from 100% energy balance and is different from the equation in > my SMPS text book.
Oh dear, I'm afraid I made a large error. I was looking at the boundary between CCM and DCM the wrong way. DCM is for large load resistance. I'm very much on the DCM side of things. (sorry) George H.
On Wednesday, April 18, 2018 at 12:16:36 PM UTC-4, George Herold wrote:
> Hi all, well I went and moded JL's flyback with the LTC3803. > I stuck the file in dropbox, but if someone can't access that I'll > be happy to copy it here. > https://www.dropbox.com/s/gd4zngedu9za4dc/GH_flyback_2.asc?dl=0 > > For the inductor I picked one of the Versa-Pak inductors with 6 coils > on one form. And made a 1:5 transformer. So I first assumed I'd be > running in continuous conduction mode. (Which may be wrong.) And went > ahead and calculated my duty cycle ratio. > V_out(300)/V_in(24) = n*D_on/D_off.. or it's on ~30% of the time. > and then the 'on' current from the 24V. ~150mA* So I picked > an inductor with a saturation current which is twice this. > (I have no idea how to pick the inductor.) Well that didn't work! > I had to increase the current a lot to get the output up to 300V. > > I needed closer to 1 amp to get the thing to work. So A different > inductor. (VPH5-0155-R) > > I must admit that if I just look at the inductor. > V = L*di/dt that at 24V and ~24uH I expect the current to increase > at about 1 amp/ usec. which is about what I see. > > So any advice about what I did wrong, or how to do this better would > be appreciated. > I should also note that there is lots of ringing in the inductor. > I'm not sure what's causing this. > > George H. > > > *This is just from 100% energy balance and is different from the equation in > my SMPS text book.
It looks fine to me George. Miscellaneous comments: The inductor ringing is normal. Putting a snubber or a damping resistor across it speeds up the simulation--I started with 50 ohms and 1nF across L1. I find it also runs well at +12V with R2=1k, and that 220pF feed-forward across the feedback divider's 3M tames your initial Vout overshoot (LTC3803, 100pF for the LTC3803-3). You could reduce ipk(L1) (operating) by using the LTC3803-3 version (50% faster), reducing the time spent in the "no energy delivery" part of the discharge cycle. That would allow you to provide the same energy with smaller pulses, delivered more often. You can reduce ipk(L1) (startup) by increasing your sense resistor. Another option: making L1 large enough to run continuous-mode drops Ipk dramatically. I prefer grounding the cold end of the output inductor, making the supply short-circuit proof. Cheers, James Arthur
On Wednesday, April 18, 2018 at 3:22:21 PM UTC-4, George Herold wrote:
> On Wednesday, April 18, 2018 at 12:16:36 PM UTC-4, George Herold wrote: > > Hi all, well I went and moded JL's flyback with the LTC3803. > > I stuck the file in dropbox, but if someone can't access that I'll > > be happy to copy it here. > > https://www.dropbox.com/s/gd4zngedu9za4dc/GH_flyback_2.asc?dl=0 > > > > For the inductor I picked one of the Versa-Pak inductors with 6 coils > > on one form. And made a 1:5 transformer. So I first assumed I'd be > > running in continuous conduction mode. (Which may be wrong.) And went > > ahead and calculated my duty cycle ratio. > > V_out(300)/V_in(24) = n*D_on/D_off.. or it's on ~30% of the time. > > and then the 'on' current from the 24V. ~150mA* So I picked > > an inductor with a saturation current which is twice this. > > (I have no idea how to pick the inductor.) Well that didn't work! > > I had to increase the current a lot to get the output up to 300V. > > > > I needed closer to 1 amp to get the thing to work. So A different > > inductor. (VPH5-0155-R) > > > > I must admit that if I just look at the inductor. > > V = L*di/dt that at 24V and ~24uH I expect the current to increase > > at about 1 amp/ usec. which is about what I see. > > > > So any advice about what I did wrong, or how to do this better would > > be appreciated. > > I should also note that there is lots of ringing in the inductor. > > I'm not sure what's causing this. > > > > George H. > > > > > > *This is just from 100% energy balance and is different from the equation in > > my SMPS text book. > > Oh dear, I'm afraid I made a large error. I was looking at the boundary > between CCM and DCM the wrong way. DCM is for large load resistance. > I'm very much on the DCM side of things. > (sorry) > George H.
Yep, you're stuck with that. For your 300V load and 300kHz, you'd need ~100mH output inductance (a 4mH-per-coil VersaPak) for CCM @ 1mA i.ripple. Faster switching would help a bit, but probably not worth sweating. Cheers, James Arthur
VPH5 seems awfully big, for only wanting 3mA out of the thing?

Fixed frequency on the controller sucks, when you don't have many options 
for the inductor.  It may be helpful to use a smaller inductance, and a 
lower ratio, to get a modestly higher primary inductance.
This approximates the VP5-0155-R:
https://www.seventransistorlabs.com/Images/GH_flyback_3.asc
Also added plausible diode parameters, a couple parasitics, and damping. 
(Hm, a ferrite bead on the diode might do a similar job with less power 
loss.  It'll need to be a fairly high Z chip type, mind.)

There are also coupled inductors of various ratios on DK, and from Coilcraft 
and others.  Or SMPS transformers, which may be cheaper than a VPH thingy 
(or even just Coilcraft's version, is cheaper than the original).

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: https://www.seventransistorlabs.com/

"George Herold" <gherold@teachspin.com> wrote in message 
news:aeaf24e2-ee49-4417-9478-332c2d99555b@googlegroups.com...
> Hi all, well I went and moded JL's flyback with the LTC3803. > I stuck the file in dropbox, but if someone can't access that I'll > be happy to copy it here. > https://www.dropbox.com/s/gd4zngedu9za4dc/GH_flyback_2.asc?dl=0 > > For the inductor I picked one of the Versa-Pak inductors with 6 coils > on one form. And made a 1:5 transformer. So I first assumed I'd be > running in continuous conduction mode. (Which may be wrong.) And went > ahead and calculated my duty cycle ratio. > V_out(300)/V_in(24) = n*D_on/D_off.. or it's on ~30% of the time. > and then the 'on' current from the 24V. ~150mA* So I picked > an inductor with a saturation current which is twice this. > (I have no idea how to pick the inductor.) Well that didn't work! > I had to increase the current a lot to get the output up to 300V. > > I needed closer to 1 amp to get the thing to work. So A different > inductor. (VPH5-0155-R) > > I must admit that if I just look at the inductor. > V = L*di/dt that at 24V and ~24uH I expect the current to increase > at about 1 amp/ usec. which is about what I see. > > So any advice about what I did wrong, or how to do this better would > be appreciated. > I should also note that there is lots of ringing in the inductor. > I'm not sure what's causing this. > > George H. > > > *This is just from 100% energy balance and is different from the equation > in > my SMPS text book.
>Yep, you're stuck with that. &nbsp;For your 300V load and 300kHz, you'd need >~100mH output inductance (a 4mH-per-coil VersaPak) for CCM @ 1mA i.ripple.
There's the Vlad memorial 'antiseptic' converter, which grabs the flyback kick and puts it in series with the output. Cheers Phil Hobbs
On Wednesday, April 18, 2018 at 6:19:43 PM UTC-4, pcdh...@gmail.com wrote:
> >Yep, you're stuck with that. &nbsp;For your 300V load and 300kHz, you'd need > >~100mH output inductance (a 4mH-per-coil VersaPak) for CCM @ 1mA i.ripple. > > There's the Vlad memorial 'antiseptic' converter, which grabs the flyback kick and puts it in series with the output.
Hmm. I remember there was such a thing, but not what it actually entails. A voltage multiplier stage would move the SMPS' duty cycle considerably in George's favor--that might help if one ever really needed CCM. Cheers, James Arthur
On Wednesday, April 18, 2018 at 4:19:18 PM UTC-4, dagmarg...@yahoo.com wrote:
> On Wednesday, April 18, 2018 at 12:16:36 PM UTC-4, George Herold wrote: > > Hi all, well I went and moded JL's flyback with the LTC3803. > > I stuck the file in dropbox, but if someone can't access that I'll > > be happy to copy it here. > > https://www.dropbox.com/s/gd4zngedu9za4dc/GH_flyback_2.asc?dl=0 > > > > For the inductor I picked one of the Versa-Pak inductors with 6 coils > > on one form. And made a 1:5 transformer. So I first assumed I'd be > > running in continuous conduction mode. (Which may be wrong.) And went > > ahead and calculated my duty cycle ratio. > > V_out(300)/V_in(24) = n*D_on/D_off.. or it's on ~30% of the time. > > and then the 'on' current from the 24V. ~150mA* So I picked > > an inductor with a saturation current which is twice this. > > (I have no idea how to pick the inductor.) Well that didn't work! > > I had to increase the current a lot to get the output up to 300V. > > > > I needed closer to 1 amp to get the thing to work. So A different > > inductor. (VPH5-0155-R) > > > > I must admit that if I just look at the inductor. > > V = L*di/dt that at 24V and ~24uH I expect the current to increase > > at about 1 amp/ usec. which is about what I see. > > > > So any advice about what I did wrong, or how to do this better would > > be appreciated. > > I should also note that there is lots of ringing in the inductor. > > I'm not sure what's causing this. > > > > George H. > > > > > > *This is just from 100% energy balance and is different from the equation in > > my SMPS text book. > > It looks fine to me George. > > Miscellaneous comments: The inductor ringing is normal. Putting a > snubber or a damping resistor across it speeds up the simulation--I > started with 50 ohms and 1nF across L1. > > I find it also runs well at +12V with R2=1k, and that 220pF feed-forward > across the feedback divider's 3M tames your initial Vout overshoot (LTC3803, > 100pF for the LTC3803-3). > > You could reduce ipk(L1) (operating) by using the LTC3803-3 version (50% > faster), reducing the time spent in the "no energy delivery" part of the > discharge cycle. That would allow you to provide the same energy with > smaller pulses, delivered more often. > > You can reduce ipk(L1) (startup) by increasing your sense resistor. > > Another option: making L1 large enough to run continuous-mode drops Ipk dramatically. > > I prefer grounding the cold end of the output inductor, making the supply > short-circuit proof. > > Cheers, > James Arthur
Hi James, Thanks... Yeah I somehow figured a bunch of that out today. I have to look more closely at some of your comments. (I mostly just copied JL's circuit and haven't tried to understand most of the values.) (It was a rare day at work, I sat down at my 'puter with book, notebook scratch pad and calc. and no one bothered me, (except for once) and when I pulled my head up it was 2:00 and I had to hurry down to the cafe, to get some lunch. So here was the idea I had driving home today. If I didn't have the transformer boost (1:5) I could use the same inductor array, as a 1:1 (3:3) with (22x9) ~200uH of inductance, I then have a short boost ~1/12.. ~0.4usec... and I should be able to get it into CCM. Hey, that Larkin guy is no dummy. :^) I wonder if a 2:4 might be better? Thanks again James. George H. ps. the 12V is good to know, the same box will have a little TEC in it, not sure exactly how much but a few volts at a few amps. My second smps circuit will be a buck. GH