Compact 300V 300mA high side switch with PNP  - Page 1
Forum section: electronics / projects

author: chipeater
author_rank: Contributor
timestamp: July 21, 2024, 07:35:50 am
content: Hi guys,Quick question. I have to imagine a “high side switch” to switch a kind of big power relay.The particularity of this relay is that you have to apply a very high voltage in the order of 250-300V for 20ms and then maintain it with a voltage of 24V ( I don't have the ref ).I need something as compact as possible, and I don't have a 10-20VDC rail for the bootstrap power supply of a Mosfet driver.It's a microcontroller that activates the coil excitation, so I need isolation between the MCU and the high voltage.The load is symbolized by resistor R1 ( 300mA ), resistors R2 and R3 are 400V resistors and R3 is a 3W resistor:I need to do more or less the same thing for the 24V rail, but without the high-voltage issue, and couple the two collectors together via diodes.Question:Do you have any criticism on this schematic ?Can you think of a more compact, more elegant, or better solution?Thank you very much

author: magic
author_rank: Super Contributor
timestamp: July 21, 2024, 09:23:51 am
content: A more compact solution would use an NPN or N-MOSFET to switch R3. But failure of such transistor may expose the MCU to high voltage, while an optocoupler provides better protection. The optocoupler must be rated to withstand 300V on the secondary, not all of them can do it, I hope you chose a suitable one.R2 is probably not really needed, and it doesn't need to withstand high voltage - it is paralleled by the BE diode of the PNP, so it will see less than 1V unless the PNP is disconnected or blown.

author: moffy
author_rank: Super Contributor
timestamp: July 21, 2024, 09:40:31 am
content: Don't forget to put a flyback/spike suppression diode across the relay coil.If you use a P-MOSFET you can use a zener to limit the gate voltage and reduce the current needed to switch the high side switch.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 21, 2024, 09:51:45 am
content: With 300V power supply you have to be careful with all the little details. PS2535 can withstand up to 350V on the Collector - Emmiter side, but how can you be sure your power supply never has peaks over that? FZT857 is also only rated to 300V, which is likely too low for your application.R3 is going to dissipate 3W and will surely overheat if you use a small resistor. Apart from power dissipation, resistors also have a maximum voltage rating and that is something you have to watch out for too in a 300V application.https://www.renesas.com/us/en/document/dst/ps2535-1-ps2535l-1-dshttps://www.diodes.com/assets/Datasheets/FZT857.pdf

author: mawyatt
author_rank: Super Contributor
timestamp: July 21, 2024, 02:22:36 pm
content: Why not use the Opto-Coupler with a HV NPN for and Opto-Coupled Darlington.Best,

author: chipeater
author_rank: Contributor
timestamp: July 21, 2024, 04:04:44 pm
content: Quote from: Doctorandus_P on July 21, 2024, 09:51:45 amThe PNP is a FZT758 so its 400V.R3 is a 400V resistor and switching the coil is just a 20ms operation so I'm not too worried.I'm actually much more worried about the transiant voltage, but I haven't found an optocoupler capable of supporting more than 350V, and TVS have a breakdown voltage that's far too wide to be used with a reasonable margin. I'm not sure what to do.

author: chipeater
author_rank: Contributor
timestamp: July 21, 2024, 04:08:19 pm
content: Quote from: mawyatt on July 21, 2024, 02:22:36 pmThat is very niceWhat is the purpose of R2? Is it to have a known potential on the base of Q1 when the phototransistor is not conducting?This avoids the need for a large HV and high power resistor.

author: mawyatt
author_rank: Super Contributor
timestamp: July 21, 2024, 04:43:35 pm
content: Quote from: chipeater on July 21, 2024, 04:08:19 pmR2 just provides a path for leakage currents from the Opto-Coupler NPN transistor and the Collector-Base leakage of Q1 when OFF, not critical 10K to 100K should be fine.Yes, R3 is eliminated.Best

author: voltsandjolts
author_rank: Unknown
timestamp: July 21, 2024, 05:20:50 pm
content: ^ Note that when OFF there could be 300V across the phototransistor, so be sure to use a part rated for 350 or 400V.https://www.mouser.co.uk/c/optoelectronics/optocouplers-photocouplers/transistor-output-optocouplers/?maximum%20collector%20emitter%20voltage=350%20V~~400%20V&rp=optoelectronics%2Foptocouplers-photocouplers%2Ftransistor-output-optocouplers%7C~Maximum%20Collector%20Emitter%20Voltage

author: profdc9
author_rank: Frequent Contributor
timestamp: July 21, 2024, 06:06:47 pm
content: You might want to consider a Darlington configuration so the optoisolator doesn't have to switch as much current, and the off and on states are in cutoff/saturated as possible.

author: MasterT
author_rank: Frequent Contributor
timestamp: July 21, 2024, 06:46:11 pm
content: I'd prefer initial circuits, as more reliable in case of capacitive load or overcurrent . Darlingtons usually are slow, and consequently load current likely to pass via optocoupler, that was not design to dissipate much power

author: mawyatt
author_rank: Super Contributor
timestamp: July 21, 2024, 07:16:43 pm
content: Quote from: voltsandjolts on July 21, 2024, 05:20:50 pmThat's no different than the original PNP based circuit shown by OP, both must sustain the supply voltage when off.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: July 21, 2024, 07:19:25 pm
content: Quote from: profdc9 on July 21, 2024, 06:06:47 pmSee #4. The NPN version the Opto-Isolator only switches the base current of Q1 plus the small amount of Vbe/R2.Best,

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2024, 07:40:00 pm
content: Quote from: chipeater on July 21, 2024, 04:04:44 pmFor the PNP design, a high voltage TVS or zener could be placed in series with the optocoupler to reduce the voltage across the optocoupler. A high value resistor then placed across the optocoupler sinks the leakage through the TVS so the voltage across the optocoupler is now the supply voltage minus the voltage across the TVS.Several TVSs in series could be placed across the optocoupler to protect it from voltage surges.A high voltage NPN or N-channel MOSFET cascode transistor could be added in series with the optocoupler. This would allow a low voltage optocoupler to be used. It also allows the optocoupler output to be current limited.I like the last option however biasing of the cascode transistor could be tricky.Alternatively, a photovoltaic optocoupler like the TLP591B can drive the base-emitter or gate-source and never see the high voltage on the collector or drain. It will supply 10 volts or 50 microamps. Switching time for a huge MOSFET is still just 1 or 2 milliseconds, so much faster in your application.

author: mawyatt
author_rank: Super Contributor
timestamp: July 21, 2024, 07:43:35 pm
content: Quote from: MasterT on July 21, 2024, 06:46:11 pmThe Darlington Transistor configuration can be quite fast, Avantek (later acquire by HP) introduced RF Darlingtions ~4 decades ago, later Mini-Circuits also offered these.Here's an old article discussing RF Darlington use.https://www.arrl.org/files/file/Technology/microwave/8702023.pdfThe Opto-Coupler Darlington shown in #4 the Opto-Coupler's output transistor's collector current is limited to Q1's base current, so Q1 will likely give up the magic smoke before the Opto-Coupler goes, in which case they'll both be toast!!If one is concerned about a capacitive load charge current then some form of current limit needs to be added, be it a emitter and/or collector resistor, or active current limit with emitter sense resistor and NPN. Here's an example.Best,

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 21, 2024, 08:57:45 pm
content: Quote from: chipeater on July 21, 2024, 07:35:50 amHow important is idle power ?If the 250-300V is somewhat stable, you can reduce the voltage stress on the opto with a zener, either series or a shunt regulator.You can get 300+V PFET, which have lower gate energy needs.You did not show any 20ms timing parts, is that done in the MCU with two pins ?If not, you might want to add a RC for the 20ms and maybe a schmitt, to keep things fast on the edges.With no schmitt, it can be simpler but you need to check the slow turn off SOAR of the HV switching device.If this has wires to the 'big relay' you might consider some current limit on the HV switching part too, so it does not instantly fail on fault.

author: MasterT
author_rank: Frequent Contributor
timestamp: July 21, 2024, 09:20:45 pm
content: Quote from: mawyatt on July 21, 2024, 07:43:35 pmNot really, BJT can pass ~1A base current before it's turn on, optocoupler in this time frame would dissipate 300 Wt.Your second circuits is a good example of what should not be done, current limiting transistor would pass all transition current via optocoupler, guaranteed failure on first try.

author: mawyatt
author_rank: Super Contributor
timestamp: July 21, 2024, 11:55:25 pm
content: Quote from: MasterT on July 21, 2024, 09:20:45 pmCare to make a serious $ wager on thatBest revisit your circuit analysis and semiconductor physics before making unqualified statements like thatCarefully analyze the circuit before jumping to unqualified statements, note the added two resistors, might save you some serious $ in case you accept the wagerBest,

author: nctnico
author_rank: Super Contributor
timestamp: July 22, 2024, 12:27:39 am
content: How about using a MOSFET output opto-coupler? These can be had with reasonable high current and high voltage handling abilities. Toshiba has quite a wide range of these.

author: mawyatt
author_rank: Super Contributor
timestamp: July 22, 2024, 12:58:42 am
content: Quote from: nctnico on July 22, 2024, 12:27:39 amThat would work and be simpler, any recommendations (are they expensive)?Best,

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 22, 2024, 06:23:31 am
content: Quote from: chipeater on July 21, 2024, 04:04:44 pmThat has poor gain at 300mA, so you need a 'better' part.Since this is a ~20ms pulse you can use a N-MOS fet with a bootstrap gate drive, that a > 20V opto coupler can manage.You can drive that opto either from a timed MCU pulse, or a RC + NPN + emitter resistor.If using a RC drive, the opto gain has some influence on the pulse, so you would pick a 'better' gain-span defined part.

author: mawyatt
author_rank: Super Contributor
timestamp: July 22, 2024, 03:44:06 pm
content: If the OP chooses the NPN version shown in #4 and #14 there's a number of available low cost HV NPNs.Here's a quickly done LTspice simulation of the NPN version mentioned with Current Limit. Since we had no HV NPN model for ~300ma, we just paralleled 4 2SCR346P LTspice models. For the Opto-Coupler we just used the LTspice model for the popular PC817A since it doesn't include a breakdown parameter for the output transistor.Note the current from the Opto-Coupler, displacing the "myth" mentioned above in #16 that's it's going to "blow up" due to the excessive 1A base current required by the output pass NPN at Turn OnNot really, BJT can pass ~1A base current before it's turn on, optocoupler in this time frame would dissipate 300 Wt.Your second circuits is a good example of what should not be done, current limiting transistor would pass all transition current via optocoupler, guaranteed failure on first try.Plots are from no Load Capacitance, Rload = 1.5K, then Rload = 1K, then Rload = 1.5K & Cload =1uF, then 10uF then lastly 100uF.Best,

author: nctnico
author_rank: Super Contributor
timestamp: July 22, 2024, 05:20:09 pm
content: Quote from: mawyatt on July 22, 2024, 12:58:42 amAs I wrote, Toshiba has many different types. But these are kind of expensive. In modern oscilloscopes these opto-MOSFETs are often used to switch between DC and AC coupling.

author: mawyatt
author_rank: Super Contributor
timestamp: July 22, 2024, 06:40:09 pm
content: Quote from: nctnico on July 22, 2024, 05:20:09 pmInteresting devices, however very slow (~1ms). Not surprising considering the semiconductor mechanisms employed, but might be highly useful where switching speed isn't required.Best,

author: chipeater
author_rank: Contributor
timestamp: July 22, 2024, 07:37:41 pm
content: Hello, thank you everyone and thank you mawyatt.I will try to optimize with other NPN. I need to make compact but I also like to take some margin.Are there any particular parameters to look at for the darlington configuration to work with the optocoupler?I don't think it's possible to be as compact with a N Mosfet solution. You need a driver with a 10-20VDC power supply for the bootstrap power supply + HV bootstrap capacitor.Just to switch the coil of a relay, I think that's excessive.

author: David Hess
author_rank: Super Contributor
timestamp: July 22, 2024, 08:21:24 pm
content: Quote from: chipeater on July 22, 2024, 07:37:41 pmThat is why I suggested a photovoltaic optocoupler like theTLP591Bwhich generates its own 10 volt supply. Only the high voltage n-channel MOSFET and the photovoltaic optocoupler are required, so 2 parts.

author: BrianHG
author_rank: Super Contributor
timestamp: July 22, 2024, 09:33:41 pm
content: Quote from: David Hess on July 22, 2024, 08:21:24 pmOne note about those photovoltaics, they need a good 30-40ma driven into the LED side to guarantee the full 10v output if your circuit is warmer than 25deg C. Other than that, everything else should work fine. If you go for a logic-level mosfet, then you can get away with driving the led at ~20ma.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 22, 2024, 09:35:34 pm
content: Quote from: David Hess on July 22, 2024, 08:21:24 pmYes, those are simple and easy to apply, and allow free choice of a MOSFET switching part, but they are not low cost.Panasonic make a very small one - APV2111Another possible part is the Photo-mos telecom type opto couplers, they have MOS parts included with the photovoltaic drivers.The volumes these are made in, mean they are cheaper than the photovoltaic drivers - lcsc have some 400V ones at 34c.They could pair with a NPN transistor, or MOSFET.High voltage NPN optocouplers tend to have poor gains ranges, most just give a min CTR of 20% or 50%, whilst lower voltage optos have tighter choices.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 22, 2024, 10:45:23 pm
content: Quote from: mawyatt on July 22, 2024, 03:44:06 pmYes, NPN Opto couplers will naturally current limit, as they are current-gain devices.That said, they still need some care as they have a puny 150mW NPN rating, and scant SOAR information.LTSpice gives optimistic CTR values, the real high voltage Optos have poor specs, they mostly just give 20% or 50% min and 600% max.That means getting enough drive for a lazy BJT/opto pair, has power envelope issues with a high gain opto, especially at current limit active.That means they will need 10mA or more LED drive, and finding high voltage NPN parts with good HFE is also difficult.Strangely, at 400V and above, the PNP FZT958 looks better than any of DiodesInc's NPN parts.An opto coupler with MOSFET output aka telecom relay types with a surge resistor+BJT may be a better drive choice.eg a GAQY216S is 600V and shows 80mA average 1A peak On current, 35 Ohms typ at 5mA IF, and 30c/500 at lcscAddit: another candidate part is something like BCW214A, which is a Dual SSR, rated to 140mA continuous and 600mA peak/100ms per channel.Parallel the outputs and series the LEDs, and it's done in one part.A part like CPC1983B has a higher price, and higher ratings.It has a curve showing 3A @ 10ms, 2A @ 100ms, 1A @1s, and 500mA continual. For 20mS it is rated > 2.5A

author: mawyatt
author_rank: Super Contributor
timestamp: July 23, 2024, 03:33:34 am
content: There's some inexpensive available (<$1) HV Darlington NPNs like theNJD25N04NJD35N04 or ST901 that should work fine as the NPN pass transistor for OP.Best,

author: oilburner
author_rank: Contributor
timestamp: July 25, 2024, 03:35:25 pm
content: Hi Mawyatt, should that be NJD35N04? (google never heard of NJD25N04)Thanks!

author: mawyatt
author_rank: Super Contributor
timestamp: July 25, 2024, 04:05:16 pm
content: Quote from: oilburner on July 25, 2024, 03:35:25 pmYeah, good catch, just corrected our posts.Thanks,Best,