Does MOSFET gate current increase with load? - Page 1
Forum section: electronics / general

author: bogdan2014
author_rank: Regular Contributor
timestamp: February 13, 2022, 09:06:35 pm
content: I've built various switching converters and I noticed that the current needed to drive power MOSFETs increases with load (at least this is what I assume). I've been in the situation where, with improper drive circuitry, the converter either couldn't handle loads or the drive transistors exceeded thermal limits.For example, a half bridge converter with a gate drive transformer and a simple BC546/BC556 totem pole driver would run perfectly fine with no load, but when the load was connected the small drive transistors would fail after a while, presumably unable to whitstand the peak currents needed to charge the gates.Am I correct in assuming this?

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 13, 2022, 09:25:46 pm
content: The gate voltage charging profile changes with load and also Vds, it makes the plateau longer. Also die Temp and gate voltage change the gate oxide characteristics and lower mosfet reliability. The HGTB reliability test tries to mimic thishttps://www.sciencedirect.com/science/article/abs/pii/S0026271421001797#!

author: ahbushnell
author_rank: Frequent Contributor
timestamp: February 13, 2022, 11:07:03 pm
content: How are you operating the device. Do you have a schematic and specs? For DC there is no gate current into a MOSFET. It's a high impedance device. If you are operating at a high frequency there is charge going in and out in switching.

author: xavier60
author_rank: Super Contributor
timestamp: February 13, 2022, 11:17:58 pm
content: With a driver transformer being used, there would be the increase in magnetizing current with duty cycle and possible saturation. Assuming that PWM is being used.I use AO branded dual MOSFETs in SO8 package is common Source configuration.With the typical half bridge and output choke topology, or PC PSU, most of the duty cycle increase occurs from no load to where choke current becomes continuous.

author: bogdan2014
author_rank: Regular Contributor
timestamp: February 14, 2022, 05:53:58 am
content: I've attached an example schematic below (only the control section). In this case, it's a two switch forward topology with a gate drive transformer. The power supply runs fine with no load but with the load attached Q1 and Q4 blow up after a short time. The transformer inductance is in the range of 470uH-1mH, and the drive signal looks fine.

author: xavier60
author_rank: Super Contributor
timestamp: February 14, 2022, 09:31:24 am
content: The specs for the UC3845 imply that the drive output voltage is limited to about 13V regardless of VCC. I have never bothered to confirm it.What is your VCC?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2022, 11:40:48 am
content: Show layout.Why intentionally throw away gate drive capacity? 3845 does 1A peak, BC845 does 100mA, maybe 200mA with drive this strong. The driver should probably have clamp diodes too, to deal with GDT inductance. Note that GDT voltage is half aux supply (minus saturation of all the transistors into it), not full peak, because of the coupling cap and flux balance.D3/D4 seem irrelevant given R16, and who knows what leakage inductance and DCR are. Maybe the GDT is a step-up ratio, I don't know. Seems unlikely to give enough drive voltage for IRF840 if it's 1:1:1.A complementary "DC restore" network on the gate side is probably a good idea.Tim

author: bogdan2014
author_rank: Regular Contributor
timestamp: February 14, 2022, 03:11:09 pm
content: QuoteI'll look at that but I think I remember seeing more than 13V with a 16-18V VCC.QuoteWell because when driving the GDT directly, the power supply can't hold any significant load, and the drive signal messes up. I double checked by supplying AUX with a separate lab power supply. When adding the drive transistors, the power supply can hold the load but they blow up after a while.QuoteThat's interesting. I'll investigate.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2022, 05:27:36 pm
content: What do you mean direct?If you don't have sufficient drive/aux power, something very basic is missing.In addition to DC restore, the GDT can be driven half-wave, preserving supply voltage for a 1:1 ratio. The flyback needs to be clamped of course, at a high enough voltage to reset the flux before next cycle, but not too high to violate gate voltage ratings of course. (Or that can be managed with other gate networks, etc.) With the 50% duty limit on this controller, this is not a problem. Take a look at typical drive circuits for 2-sw forward.Under startup conditions, it needs to be overcharged enough to get the output up to nominal before it runs out of poop. The UC3844 is better suited to self-power designs, starting at 16V supply and stopping at 10. If aux is separately powered, then it simply needs to be enough to run the thing (some 10s mA).Tim

author: bogdan2014
author_rank: Regular Contributor
timestamp: February 15, 2022, 06:21:44 am
content: QuoteDirectly by the control chip, without any buffer/drive circuit.QuoteYou mean flyback style?Below are some waveforms. First is the chip output, second is the GDT output (driven directly by the chip via a coupling cap. Peak voltage is 18V with the same VCC, but on the GDT peak voltage is 12.6V due to the DC restore circuit. So that might be what I missed. A 1:1:1 ratio won't work unless I increase VCC to something like 24V.

author: bogdan2014
author_rank: Regular Contributor
timestamp: February 15, 2022, 09:19:43 am
content: I replaced the main transformer with a bulb to see how it reacts to a resistive load. First picture is the gate waveform without a load, second is with a load. The bulb would usually draw around 4A, but in this case it only draws around 1A and the switching transistors get hot. So there's definitely something with the drive circuit.The GDT is driven directly by the control chip and there's series caps on the primary and secondaries.

author: xavier60
author_rank: Super Contributor
timestamp: February 15, 2022, 09:40:24 am
content: Increasing MOSFET load current will shift the Gate plateau voltage up a bit. Because the Gate drive voltage transitions take so little time compared to the switching cycle time, any increase in drive current will be miniscule.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2022, 01:21:43 pm
content: Quote from: bogdan2014 on February 15, 2022, 06:21:44 amY...I hesitate to say 'yes', because "flyback" implies the consequent action on the backswing (after turn-off). This would be "forward" (gate is on while primary drive is on). If you mean just in terms of primary drive circuit, yes, it's the same, just a transistor (open collector/drain) driving the winding hanging from +V, and then damping or clamping to deal with the flyback (reset pulse).Unlike a forward converter proper, little power is delivered by it, and not into a rectifier and inductor load, but just for the voltage into the gate. (The backswing can be clipped by diode or zener, in case it might be prone to overvoltage the gate. Like I said, check out typical gate drive networks for this. The common one is something like, a series diode to charge the gate when turning on, while blocking discharge, then a PNP with a resistor from winding to base, emitter to gate, collector to source, to enhance turn-off while also clamping it to Vgs ~ 0.)QuoteYeah, the baseline is always in the middle because the inductor has to balance flux, and the cap decouples the DC voltage to allow this.Note that the sharp cornery bits, following rise/fall, momentarily exceed +V/GND. This is the GDT's magnetizing current pushing the voltage beyond the rails, momentarily. It looks like the bottom phase is pretty stable, and also lower resistance; this is probably a saturated NPN to GND, so has reasonably low resistance even when reversed. (BJTs work just fine under inverted operation, albeit hFE is usually lower. So, up to some modest reverse current, we expect a similar voltage drop as Vce(sat). And, considering it's a 1A NPN, it can probably stay pretty well clamped to a couple 100mA in reverse.) The top phase is less stable, an NPN emitter follower I believe (or even Darlington), so has two things: a weak diode into +V (hence the extra overshoot), and a relatively high voltage drop when forward biased (sourcing current into the load). Hence the much more noticeable flyback sort of blip, then stepping down to a softer (slope-ier) conduction phase.Excess voltages are probably best to avoid, so I would recommend adding a schottky diode from OUT to +V. Something like MBR0530 is probably fine, maybe even BAT54. One from GND to OUT also wouldn't be a bad idea.This becomes irrelevant when half-wave driving the transformer with another transistor, of course. (BJT or MOS is suitable for this; probably MOS for convenience. Note that a BJT needs a base current limiting resistor, and B-E resistor to ensure fast turn-off.)Tim