secondary turn-on

Hi I am using the HIP4081 driving a H bridge with 4 STP24NF10 fets- Gate resistors are 10R in high side fets, 33R in low side, all drive R's have 1n4148 discharge diodes to speed turn off- circuit functions well -except for a 5n pulse on the low fet coincident with the turn on of the high fet- PCB is layed out to full guidelines from harris including laying gate traces with source feed on underside as plane to reduce the chance of this phenomina-

any usefull ideas on reducing this- unit works well but this is obviously wasting power and heating the low side fet (although even without sink its mostly cold)

Regards

Anthony C Smith

I doubt that's your problem, not if you were to use Schottky pulldown diodes to control the off FET's anyway. Did you provide a delay using the '4081 HDEL and LDEL resistors (pins 8, 9) to prevent shoot-through?

--
 Thanks,
    - Win

with

harris

If the little pulses are large enough to cause dV/dt induced cross conduction (have you actually measured the cross conducting current, or have you merely observed small less than threshold blips appearing on the low side MOSFET gates?), then there are a number of things you might try.

This document:

Would have you believe the gate resistor paralleled with diode only provides incremental improvement over just the gate resistor alone (see figure 12 and related text in the document). From my personal experience with dV/dt induced cross conduction I would tend to agree. The document claims the circuit of figure 13 is superior. I tend to agree, but I think the best solution would be if MOSFET gate driver IC manufacturers provided two separate pins for each gate drive output. One pin for sinking current and one pin for sourcing current. That way you could use separate valued gate drive resistors for sinking and sourcing. It seems in most power MOSFET applications (especially true in H-bridges) turn on should be a fair amount slower than turn off for optimal performance.

In addition to trying the circuitry of figure 13 you might also consider applying negative gate drive, or perhaps keeping the same circuit you are currently using (gate resistor in parallel with diode) but increasing the upper MOSFETs' gate resistor values. If cross conduction really is occurring, this can sometimes dramatically reduce total MOSFET heating even though it makes turn on slower. Of course, this isn't optimal from a performance standpoint since you probably want really fast turn on and turn off without any dV/dt induced cross conduction, but sometimes the added gate drive complexity may not be worth it (especially since fast turn on/off worsens the EMI situation).

thanks for the input- Can you reccommend any decent fets for the job? switching frequency is

384KHz, power is 200W into 1R @32V max, 22V min the H bridge drives a 40u common mode choke followed by a 3rd order butterworth filter at 50KHz regards Anthony

I just checked the data- original design used RFP22N10 fairchild devices- this ST part was the nearest equivalent, but with better (lower) rds on and a lower gate charge- gate threshold voltage is the same 2-4V- the lower gate charge reduced the quiecent current of the amp- which is good as it has to run on batteries for backup. while searching I found a p fet optimised for Cgs/Cdg of 0.7 to minimise secondary turn on- has anyone used additional gate C to prevent this? regards Anthony

How did you make this measurement (keep in mind L dI/dt)?

--
 Thanks,
    - Win

-snip-

Using 68K for HDEL and LDEL- using the DSO I can see the delay so this is not the problem- I also increased the values to no avail- I am trying the diodes now Regards Anthony

-snip-

have

No I have measured the current -5ns 50A flowing hence wanting to remove it!

Thanks for the link most useful

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Thats why I have the different values on upper/lower halves of the bridge- this reduced ringing to virtually nothing and boosted performance

Your so called non-inductive sense resistor is still inductive. I'm not sure what you mean by 5ns 50A, but suppose a current rise time from 0-50A in

5ns. This suggests a dI/dt of 10,000,000,000 Amps/second. So how much inductance would produce 0.75V with that kind of current ramp? Well, E=L*dI/dt, so in other words a mere 75 picoHenries would produce 0.75V with such a high current ramp rate. Given a vague rule of thumb of around 1nH per millimeter of trace length, it is very probable your "non-inductive" resistor has way more than 75pH of inductance (no matter its actual construction). Additionally it would be necessary for your scope probe to be connected impractically close to the resistor to avoid including 75pH of inductance in the measurement.

So the basic idea is it is difficult to actually measure the cross conduction current with any reasonable expectations of accuracy. By the sounds of it you probably are getting cross conduction, but it is probably not as severe as you think it is.

Thanks, Fritz, for making my point so eloquently. Anthony's inductance will no doubt be much more than 75pH, or even 10nH. For example, power MOSFET internal lead wiring is typically 5 to 10nH. If we assume 5nH total, a 0.75V drop implies a slow 150mA/ns current rate-of-change.

It's not clear Anthony is seeing shoot-through currents; there are many other sources of impulse noise during switching that may be visible at the FET's source lead. For example, the high capacitive gate currents go through that path. One insidious severe noise source comes from fast current snap-off in the FET's substrate diode after its reverse-recovery time is finished. The primary way to tell this is happening is presence of severe switching noise. This situation will arise whenever the FET is off and its diode is made to conduct, say caused by flyback from the inductance in Anthony's output filter, "H bridge drives a 40u common mode choke followed by a 3rd order Butterworth filter at 50kHz." In extreme cases the source-voltage bounce can be severe enough to damage the FET by (internally) exceeding its gate-voltage limit for a few ns. This I know from sad experience, and from careful measurement with my superior probes (I mention probes, because its VERY easy to be mislead when taking such measurements, e.g. move the probe over to ground on the other side of the sense resistor and observe the zero-volt signal... and ditto elsewhere).

--
 Thanks,
    - Win

It measures as a 0.75V peak pulse across a 0R015 non inductive sense resistor between the bottom of the bridge and true ground- both paths are less than 5mm long and over 5mm wide to avoid skin effect problems regards Anthony

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Thanks for the explaination- I am trying to fix a problem on a current board any patch ideas? this is driving me up the wall- I was trying to lower the THD and this spike is everywhere to some extent reagrds Anthony

You could increase the upper fet gate current turn-on limiter. You obviously have incremental values to play with before you hit the same value used on the lower part.

You haven't yet indicated whether the use of schottkies did anything to your measurements.

RL

Its definately secondary turn on, albiet maybe smaller than I first thought the above is not realy practical as its not a smps, its a discreet modulator driving the HIP for an audio amplifier and the sense resistor is part of the I limit system. regards

I have been trying this and it appears to be working

Strangely they did not improve things thanks for the input

I think I am getting to the problem- not quite where I thought it was, the bridge is a phase shift type, with 0V in both bridges have a common mode switching frequency, and switch together, +v one bridge duty cycle increases, the other decreases,-v the opposite. By removing the load and applying a DC offset it appears that the pulse is comming from the oposite bridge half, and appears to be from the half under measurement when at 0V or small signal input.

Got the T shirt on this one- all measurements are relative, using stored values and the same probe with the same reference ground for the probe, also using 100MHz DSO to capture as much as possible. I have had problems with this part and failures, so in this design I have implimented the 1R series resistor from mid point to BHS and a UF4003 from this point to BLO as noted in a Harris aps note

Whoa. You are driving a common-mode choke with a phase-modulated H bridge? The common mode fundamental output component of a phase modulator is extremely large.

Are you sure you are not driving your sources and drains above and below their power rails? Is current really zero before the second conduction event?

RL

Hi Anthony,

A perhaps stupid question - is what you are seeing actually gate current? If possible, re-route the lower FET gatedrive such that gate charge and discharge current does not flow thru the sense resistor (easy to do if you have a separate power gnd on the smps chip, or a transistor buffer).

Cheers Terry

The choke removes most of the common mode component, the following filter then only works on the diffential signal.

There is currently a 3V overshoot on the bridge output, no undershoot (or negligable) and all four devices are snubbed with 10R/1n, as well as the cm choke to reduce ringing. from the data the devices should cope with this easily-or am I missing something?

scaling of CM Choke- I have scaled the current for the choke at 10A (I need

7A through the choke to drive the audio) as the common mode signal should result in no net ampere turns does anyone have any experiance with this?

thanks

Anthony