Logic level HV fet...

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Hi there. Unfortunately the pickings for 200V or more logic level MOSFETs is exceedingly slim. For the most part your only potentially feasible choices I'm aware of are the IRL620, IRL630, IRL640 (TO-220 devices) and the D2Pak versions of those same parts: IRL620S, IRL630S, IRL640S. Here is the datasheet for the IRL630S which is a 200V 0.4 ohm Rds(on) D2Pak part.

Digikey sells all of these device types. They aren't really very good, noticeably inferior to their standard gate drive newer cousins.

The LTC1872 is a nice part, but the switching frequency is really unreasonable for this application. Efficiency would be hideous at 550kHz operation, no matter your MOSFET selection. Without doing any major calculations before spouting off conclusions, I would bet efficiency would be so low as to preclude the use of an SMD MOSFET for thermal reasons alone, despite your low 3.6W output power. In addition to abysmal efficiency, you could also expect tremendous EMI production, likely way beyond FCC limitations. If you listen to the radio at all, you quite possibly might find that all but the most powerful AM and FM band stations will have become too static filled to listen to.

The problem is high dV/dt radiates. A little 3.3V to 5V step up converter can operate at high switching frequency with very fast transitions without producing very much EMI. The drain of the MOSFET would only swing from

I would strongly recommend using something much closer to 20kHz as your switching frequency. Make sure to use discontinuous conduction mode since the control loop gain gets really large (IE: impossible to make stable) for large step up ratios (which 9V to 180V certainly qualifies) in continuous conduction mode. With a 20kHz switching frequency and 9V input an inductor value of vaguely around 150uH would seem to be reasonable.

John

Hi,

I'm looking for a 200 - 250V FET with low Qg, low RDSon (only needs maybe 3A peak Ids, but low RDSon for efficiency) and LOGIC LEVEL GATE for use in an SMPS that only has a 9V supply - the SMPS is in boost configuration to give

Any ideas would be appreciated - I've not had a lot of luck so far - I know its a tall order. An SMD footprint would also be good.

Thanks

Mike

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Hi Fritz,

Thanks for the reply. Normally, I use a MAX1771 which runs at 50kHz and gives me about 87% efficiency in this configuration - really a very nice chip.

However, getting the MAX chips in Europe and elsewhere is often a pain, and I was looking at emulating its performance using LTspice when an LT FAE mentioned their devices, which are emulated for you in LTspice, and he suggested I look at those as well, hence this investigation.

I agree with everything you say about the LT chip - it is a very nice device in a very small package, but the maximum efficiency I've been able to get is in the order of 65%.... Also, unlike the MAX1771, the LT device can only handle up to 9.8V in, which really means logic level FETs - if I was going at add a charge pump or whatever for the gate drive, I might as well use the MAX device which is more forgiving anyway...

Ho, hum... just a thought...

Thanks

Mike

Hi Fritz, This is a great application for running a simple boost in Zero Voltage Switching mode. Add more capacity in parallel with the FET to control (resonate) rise and fall times to about 200nS. Operate the PWM in constant "off time" and current controlled "on time". Most control IC can be configured for this. I use this mode of operation so I can lisiten to the radio as I check it out. Piece of cake. Harry

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Hey now that is an interesting part. I wonder how much it costs...

I agree, ZVS would be very nice for this application. This is a good idea.

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The 2SK2350 is 200V, 0.26ohms, with 4V gate drive.

Siliconix Si4490DY Nchannel 200V 0R09 in SO8, is characterized with

RL

It's a boring part. Yawn.

It appears the 0.26ohm rating is with 10V drive. That's a boring part. Nothing special. Yawn.

The last time I was fooling with 150V parts, top of the line SO8 packages were about US$ 1.70, when they hit their intended volume. Small quantities and early parts could be three times that.

Never inquired about Si4490. The 150V parts were 0R05 or less. RL

Yes but only on a casual reading. Look at the graphs. This fet has very nearly the same Ron at a 5V gate. This is in sharp contrast to many Fets advertised as being 'logic-level' when in actual fact they are barely starting to move, hence incapable of doing any real work at this drive level.

Go on then, suggest a logic level fet, easily available at a reasonable price, that should be preferred for this switching job. An insulated case type would be nice. :-). regards john

No, I prefer it with the skin.

Reality is such a bore!

Shall I peel you a grape?

RL

Hello John,

If you look at the Vgs versus Vds graph in the middle of page 3 there is less than a volt of head room at 5V gate drive before Vds shoots up and potentially turns the device into a pile of ash. Too close for comfort to me.

That's just the problem, not much out there. It is an issue I run into a lot and often end up with bipolar transistors. What really drives me up the wall is a glitzy ad for some new FET, saying it is guaranteed 3V or something and then in the fine print I find that was for 1mA and Vds=1V. At least one can then crumple the data sheet, form a ball and smack it into the bin across the room. Or use it to light the wood stove.

Regards, Joerg

Not at all.

Are we looking at the same datasheet? I don't see plots of Rds vs Vgs, and trying to infer Rds(on) at 4 or 5V from the draftsman's version of the characteristic curves is usually a futile exercise.

I know they're not common, but PartMiner's parametric search of 200V mosfets turns up 107 parts that have Rds(on) specified at 5V or lower. The o.p. may wish to avoid choosing an overly-large FET, to benefit from lower gate capacitance.

Some logic-level 200V TO-220 n-channel mosfets

mosfet Rds(on) @ Vgs Ciss -------- ------ --- ------ IRL620A 0.8 5V 330pF IRLS620A 0.8 5 330 insulated case :>) FQP7N20L 0.78 5 390 2sk3215 0.55 4 410 IRL630 0.4 5 1100 old part, higher capacitance IRLi630g 0.4 5 1100 old part, high cap, insulated IRL630A 0.4 5 580 IRLS630A 0.4 5 580 insulated case :>) FQE10n20LC 0.39 5 375 insulated case :>) 2sk3214 0.19 4 1100 2sk3160 0.19 4 1100 insulated case :>) IRLi640g 0.18 5 1800 old part, high cap, insulated IRL640A 0.18 5 1310 IRLS640A 0.18 5 1310 insulated case :>) FQP19n20L 0.15 5 1700 FQP34n20L 0.08 5 3000 FQPF34n20L 0.08 5 3000 insulated case :>)

In this instance, there's little reason to use parts specified at 10V and guess their properties at 5V or less. BTW, when someone tells me they're using a 9V supply, I assume a 9V battery, which means 5 or 6V is more reasonable minimum operating voltage.

With a few more added...

mosfet Rds(on) @ Vgs Ciss comments -------- ------ --- ------ --------------- IRL620A 0.8 5V 330pF IRLS620A 0.8 5 330 insulated case FQP7N20L 0.78 5 390 2sk3215 0.55 4 410 IRL630 0.4 5 1100 old part, higher capacitance IRLi630g 0.4 5 1100 old part, high cap, insulated IRL630A 0.4 5 580 IRLS630A 0.4 5 580 insulated case FQE10n20LC 0.39 5 375 nice low-cap, TO-126, insulated fqp10n20L 0.38 5 640 fqd10n20L easier to get 2sk3214 0.19 4 1100 2sk3160 0.19 4 1100 insulated case IRLi640g 0.18 5 1800 old part, high cap, insulated IRL640A 0.18 5 1310 IRLS640A 0.18 5 1310 insulated case FQP19n20L 0.15 5 1700 FQPF19n20L 0.15 5 1700 insulated case FQP34n20L 0.08 5 3000 FQPF34n20L 0.08 5 3000 insulated case

Fairchild's amazing low-gate-charge FQE10n20LC seems hard to find. But the rest of the Fairchild '10n20L series is still attractive. Mouser stocks most of the SMT parts in the line, for example the FQD10n20L costs $0.63 qty 100.

BTW, considering newer-style packages dramatically increases the number of logic-level FET choices available.

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Vds=1V.

Somehow I see the contrast between high tech electronics and heating with a wood stove as just too great. :-)