I became discouraged of finding a suitable low-capacitance zener to protect the
MOSFET gates in my high-voltage amplifier designs. (The protection may be
optional, but one sleeps better with it in place.) Even the 50uA-rated
low-current zener diodes are in fact larger-die types, with high capacitance.
For example, the 7.5-volt MMSZ4693 has about 130pF at zero volts.
My solution was to use a Diodes, Inc. D1213A-01WS TVS device, which has a 6 to
10V breakdown and an amazing low 1pF of capacitance. But this can carry limited
current in the forward direction, so I added a BAT54WS Schottky diode in
parallel. Both devices are in small SOD-323 packages. Total capacitance, 11pF
at their zero volt maximum, which doesn't add too awfully much to the 38pF Ciss
of the small high-voltage power MOSFETs I'm using.
Or maybe one of you has discovered a better solution?
--
Thanks,
- Win
fast, small low-capacitance zener to protect MOSFET gates
Started by ●December 11, 2018
Reply by ●December 11, 20182018-12-11
On 12/11/18 9:25 AM, Winfield Hill wrote:> I became discouraged of finding a suitable low-capacitance zener to protect the > MOSFET gates in my high-voltage amplifier designs. (The protection may be > optional, but one sleeps better with it in place.) Even the 50uA-rated > low-current zener diodes are in fact larger-die types, with high capacitance. > For example, the 7.5-volt MMSZ4693 has about 130pF at zero volts. > > My solution was to use a Diodes, Inc. D1213A-01WS TVS device, which has a 6 to > 10V breakdown and an amazing low 1pF of capacitance. But this can carry limited > current in the forward direction, so I added a BAT54WS Schottky diode in > parallel. Both devices are in small SOD-323 packages. Total capacitance, 11pF > at their zero volt maximum, which doesn't add too awfully much to the 38pF Ciss > of the small high-voltage power MOSFETs I'm using. > > Or maybe one of you has discovered a better solution? > >The Central Semi CSL05 looks decent at 1.2 pF, but it has a series diode to prevent forward current. How about the ESD0P2RF-02LS at 0.25 pF? Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●December 11, 20182018-12-11
Phil Hobbs wrote...> >On 12/11/18 9:25 AM, Winfield Hill wrote: >> I became discouraged of finding a suitable low-capacitance zener to protect >> MOSFET gates in my high-voltage amplifier designs. (The protection may be >> optional, but one sleeps better with it in place.) Even the 50uA-rated >> low-current zener diodes are in fact larger-die types, with high capacitance. >> For example, the 7.5-volt MMSZ4693 has about 130pF at zero volts. >> >> My solution was to use a Diodes, Inc. D1213A-01WS TVS device, which has a >> 6 to 10V breakdown and an amazing low 1pF of capacitance. But it can >> only carry limited current in the forward direction, so I added a BAT54WS >> Schottky diode in parallel. Both devices are in small SOD-323 packages. >> Total capacitance, 11pF at their zero volt maximum, which doesn't add too >> awfully much to the 38pF Ciss of my small high-voltage power MOSFETs. >> >> Or maybe one of you has discovered a better solution? > > The Central Semi CSL05 looks decent at 1.2 pF, but it has a series diode > to prevent forward current. How about the ESD0P2RF-02LS at 0.25 pF?Missed the CSL05, SL05, a typo? Anyway, yes, dozens of amazing low- capacitance TVS protection devices available, handling high peak currents for a few microseconds. But my designs (similar to AoE III Fig 3.111, page 209) must handle 150mA continuous forward current, ordinary zeners can do it just fine. That's what the added Schottky diode is for. It contributes most of the capacitance, but increasing the node capacitance from 38pF to 50pF is an acceptable penalty. Maybe some small unipolar TVS can handle that, 10pF capacitance max? -- Thanks, - Win
Reply by ●December 11, 20182018-12-11
On 11 Dec 2018 06:25:40 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:>I became discouraged of finding a suitable low-capacitance zener to protect the >MOSFET gates in my high-voltage amplifier designs. (The protection may be >optional, but one sleeps better with it in place.) Even the 50uA-rated >low-current zener diodes are in fact larger-die types, with high capacitance. >For example, the 7.5-volt MMSZ4693 has about 130pF at zero volts. > >My solution was to use a Diodes, Inc. D1213A-01WS TVS device, which has a 6 to >10V breakdown and an amazing low 1pF of capacitance. But this can carry limited >current in the forward direction, so I added a BAT54WS Schottky diode in >parallel. Both devices are in small SOD-323 packages. Total capacitance, 11pF >at their zero volt maximum, which doesn't add too awfully much to the 38pF Ciss >of the small high-voltage power MOSFETs I'm using. > >Or maybe one of you has discovered a better solution?You could put a schottky in series with a zener. The first couple of pulses will charge up the zener capacitance. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●December 11, 20182018-12-11
On Tuesday, December 11, 2018 at 3:15:57 PM UTC-5, John Larkin wrote:> On 11 Dec 2018 06:25:40 -0800, Winfield Hill > <hill@rowland.harvard.edu> wrote: > > >I became discouraged of finding a suitable low-capacitance zener to protect the > >MOSFET gates in my high-voltage amplifier designs. (The protection may be > >optional, but one sleeps better with it in place.) Even the 50uA-rated > >low-current zener diodes are in fact larger-die types, with high capacitance. > >For example, the 7.5-volt MMSZ4693 has about 130pF at zero volts. > > > >My solution was to use a Diodes, Inc. D1213A-01WS TVS device, which has a 6 to > >10V breakdown and an amazing low 1pF of capacitance. But this can carry limited > >current in the forward direction, so I added a BAT54WS Schottky diode in > >parallel. Both devices are in small SOD-323 packages. Total capacitance, 11pF > >at their zero volt maximum, which doesn't add too awfully much to the 38pF Ciss > >of the small high-voltage power MOSFETs I'm using. > > > >Or maybe one of you has discovered a better solution? > > You could put a schottky in series with a zener. The first couple of > pulses will charge up the zener capacitance.Huh, I was going to suggest that, but I figured the series schottky would be forward biased, and so have a lot of capacitance. Is that 'wrong' thinking on my part? George H.> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●December 11, 20182018-12-11
On Tue, 11 Dec 2018 13:15:44 -0800 (PST), George Herold <gherold@teachspin.com> wrote:>On Tuesday, December 11, 2018 at 3:15:57 PM UTC-5, John Larkin wrote: >> On 11 Dec 2018 06:25:40 -0800, Winfield Hill >> <hill@rowland.harvard.edu> wrote: >> >> >I became discouraged of finding a suitable low-capacitance zener to protect the >> >MOSFET gates in my high-voltage amplifier designs. (The protection may be >> >optional, but one sleeps better with it in place.) Even the 50uA-rated >> >low-current zener diodes are in fact larger-die types, with high capacitance. >> >For example, the 7.5-volt MMSZ4693 has about 130pF at zero volts. >> > >> >My solution was to use a Diodes, Inc. D1213A-01WS TVS device, which has a 6 to >> >10V breakdown and an amazing low 1pF of capacitance. But this can carry limited >> >current in the forward direction, so I added a BAT54WS Schottky diode in >> >parallel. Both devices are in small SOD-323 packages. Total capacitance, 11pF >> >at their zero volt maximum, which doesn't add too awfully much to the 38pF Ciss >> >of the small high-voltage power MOSFETs I'm using. >> > >> >Or maybe one of you has discovered a better solution? >> >> You could put a schottky in series with a zener. The first couple of >> pulses will charge up the zener capacitance. >Huh, I was going to suggest that, but I figured the series schottky >would be forward biased, and so have a lot of capacitance. >Is that 'wrong' thinking on my part?It wouldn't be forward biased much unless the zener were conducting, which would mean it's protecting the fet. One could add a resistor to keep the zener conducting, so that the schottky was usually reverse biased. Or, equivalently, anchor the schottky to some power supply. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●December 11, 20182018-12-11
John Larkin wrote...> > You could put a schottky in series with a zener. The first > couple of pulses will charge up the zener capacitance.Interesting idea! -- Thanks, - Win
Reply by ●December 11, 20182018-12-11
On 11 Dec 2018 13:43:32 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:>John Larkin wrote... >> >> You could put a schottky in series with a zener. The first >> couple of pulses will charge up the zener capacitance. > > Interesting idea!I looked at the AoE page and see that this is a linear amp, not the Pockels cell driver. So charging the zener isn't as obvious. I like to use PV optocouplers as gate drivers, because they float on the fet source and don't have the huge gate over-voltage hazards. https://www.dropbox.com/s/pwi91ext9cclhzz/Gate_Driver_3.JPG?dl=0 With the schottky+zener, you could use a PV to keep the zener on, but that may be overkill. I guess the output fet never gets too close to the V+ rail, so a resistor from V+ to the zener could keep the zener conducting, so the schottky is usually back-biased and low capacitance. Higher voltage zeners will zener at nanoamps current, so the resistor value is reasonable, megohms. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●December 11, 20182018-12-11
On 12/12/18 3:05 am, Winfield Hill wrote:> Phil Hobbs wrote... >> >> On 12/11/18 9:25 AM, Winfield Hill wrote: >>> I became discouraged of finding a suitable low-capacitance zener to protect >>> MOSFET gates in my high-voltage amplifier designs. (The protection may be >>> optional, but one sleeps better with it in place.) Even the 50uA-rated >>> low-current zener diodes are in fact larger-die types, with high capacitance. >>> For example, the 7.5-volt MMSZ4693 has about 130pF at zero volts. >>> >>> My solution was to use a Diodes, Inc. D1213A-01WS TVS device, which has a >>> 6 to 10V breakdown and an amazing low 1pF of capacitance. But it can >>> only carry limited current in the forward direction, so I added a BAT54WS >>> Schottky diode in parallel. Both devices are in small SOD-323 packages. >>> Total capacitance, 11pF at their zero volt maximum, which doesn't add too >>> awfully much to the 38pF Ciss of my small high-voltage power MOSFETs. >>> >>> Or maybe one of you has discovered a better solution? >> >> The Central Semi CSL05 looks decent at 1.2 pF, but it has a series diode >> to prevent forward current. How about the ESD0P2RF-02LS at 0.25 pF? > > Missed the CSL05, SL05, a typo? Anyway, yes, dozens of amazing low- > capacitance TVS protection devices available, handling high peak > currents for a few microseconds. But my designs (similar to AoE III > Fig 3.111, page 209) must handle 150mA continuous forward current, > ordinary zeners can do it just fine. That's what the added Schottky > diode is for. It contributes most of the capacitance, but increasing > the node capacitance from 38pF to 50pF is an acceptable penalty.Can you tune out the Schottky diode's capacitance with a small series L? Is there anything like a string of PIN diodes in a single package? I've just used Skyworks SMP1330 to protect an RF LNA. The combination of low capacitance (0.7pF) and ability to sink power (almost 1 watt) is remarkable. If I understand it, as the device approaches conduction the I layer thins, increasing capacitance rapidly, so it becomes "conductive" even before the diode turns on. Clifford Heath.
Reply by ●December 11, 20182018-12-11
On Tuesday, 11 December 2018 23:03:03 UTC, Clifford Heath wrote:> On 12/12/18 3:05 am, Winfield Hill wrote: > > Phil Hobbs wrote... > >> > >> On 12/11/18 9:25 AM, Winfield Hill wrote: > >>> I became discouraged of finding a suitable low-capacitance zener to protect > >>> MOSFET gates in my high-voltage amplifier designs. (The protection may be > >>> optional, but one sleeps better with it in place.) Even the 50uA-rated > >>> low-current zener diodes are in fact larger-die types, with high capacitance. > >>> For example, the 7.5-volt MMSZ4693 has about 130pF at zero volts. > >>> > >>> My solution was to use a Diodes, Inc. D1213A-01WS TVS device, which has a > >>> 6 to 10V breakdown and an amazing low 1pF of capacitance. But it can > >>> only carry limited current in the forward direction, so I added a BAT54WS > >>> Schottky diode in parallel. Both devices are in small SOD-323 packages. > >>> Total capacitance, 11pF at their zero volt maximum, which doesn't add too > >>> awfully much to the 38pF Ciss of my small high-voltage power MOSFETs. > >>> > >>> Or maybe one of you has discovered a better solution? > >> > >> The Central Semi CSL05 looks decent at 1.2 pF, but it has a series diode > >> to prevent forward current. How about the ESD0P2RF-02LS at 0.25 pF? > > > > Missed the CSL05, SL05, a typo? Anyway, yes, dozens of amazing low- > > capacitance TVS protection devices available, handling high peak > > currents for a few microseconds. But my designs (similar to AoE III > > Fig 3.111, page 209) must handle 150mA continuous forward current, > > ordinary zeners can do it just fine. That's what the added Schottky > > diode is for. It contributes most of the capacitance, but increasing > > the node capacitance from 38pF to 50pF is an acceptable penalty. > > Can you tune out the Schottky diode's capacitance with a small series L? > > Is there anything like a string of PIN diodes in a single package? > > I've just used Skyworks SMP1330 to protect an RF LNA. The combination of > low capacitance (0.7pF) and ability to sink power (almost 1 watt) is > remarkable. If I understand it, as the device approaches conduction the > I layer thins, increasing capacitance rapidly, so it becomes > "conductive" even before the diode turns on. > > Clifford Heath.I wondered about a diode string instead of a zener, but it ups cost & adds bulk. NT
