Voltage rise on gate / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Voltage rise on gate

previous 1 2 3 4 next

Move Thread

LAN_403

furnace

Sun Feb 08 2015, 04:10AM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108

The mosfets are ok, because when I remove the zeners it turns back on again but than I get the high voltage on the gate. The high voltage reading is from the dc blocking cap being charged.

furnace

Wed Feb 11 2015, 12:24AM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108

I take it that this is normal for the half bridge to have a rise on gate of the high side?

Antonio

Wed Feb 11 2015, 12:57AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

furnace wrote ...

The scope is grounded through a cap and the probes are directly on the gate of the mosfets.
But anyway this is not to be considered as this is not the source of my problem. The problem is that the voltage of my high side mosfet is rising on the gate or it seems to build up a charge.

This would short-circuit the output of the bridge to the negative rail through the ground probes of the oscilloscope, if both are connected at the same time. The risk of extensive destruction is high... If you connect the ground of the oscilloscope to the output of the bridge, there are high-frequency signals everywhere, with all kinds of parasitic effects. If a capacitor is grounding the oscilloscope, it is across the output of the bridge. As a security measure with these measurements, connect the ground probe of the oscilloscope through a small 100 ohms resistor. If something is wrong the resistor burns, and not something more expensive.

furnace

Wed Feb 11 2015, 10:56AM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108

Thanks Antonio, this didn't solve the problem but my low side mosfet is behaving better, it seemed to short on the gate of the lower mosfet.
Any advise on the voltage rise of my high side mosfet?

Antonio

Wed Feb 11 2015, 12:47PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

D5 and D6 should not exist, or at least should have small resistors in parallel, and be not zeners, but fast diodes. Their function would be is to accelerate the turnoff of the mosfets, but the effect is small. With the diagram as shown, the mosfets would not turn on. If the zeners are reversed they would not turn off.
Anyway, you can't measure directly the vgs voltage of the high-side mosfet without causing many loading problems, if not a straight short circuit. Try to observe the gate and the source with grounded oscilloscope probes and subtract the signals. The ground terminals of the probes can only be connected to ground.

furnace

Wed Feb 11 2015, 01:52PM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108

Yep! I figured out the zeners. I only have 15ohm resistors in series with gate and the signal is perfect. Can you please explain more in detail what you mean that I should observe the gate and source With grounded scope probes and subtrcact The signals?
At the moment my probe is directly on the high side gate and ground of the scope is on the main ground with 200ohm resistor.

Antonio

Wed Feb 11 2015, 02:18PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Many oscilloscopes have a function that subtracts the signals on both probes, or invert one and add. If yours don't have it, observe both signals, gate and source, with both probes grounded and subtract visually, what is probably better.

furnace

Wed Feb 11 2015, 02:45PM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108

I have an inverted switch on my scope. Everything looks ok, it's just that my low mosfet is getting hotter than high one and the high signal is still rising when as I increase power on the variac.

Wolfram

Wed Feb 11 2015, 08:52PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

If you're measuring without the top MOSFET source grounded, then you're measuring the sum of the lower MOSFET drain voltage, and the upper MOSFET gate voltage. As the drain voltage increases when you apply power with the variac, the gate voltage will appear to rise when it isn't

If you are measuring with the top MOSFET source grounded, then you're either shorting out your bridge or putting live mains on the scope chassis (only possible if the scope is not grounded and you're not measuring the bottom MOSFET gate voltage at the same time).

furnace

Tue Feb 17 2015, 06:18AM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108

I'm blowing my switches, I've even had different mosfets on the bridge and tried igbts, the igbts get very hot when I use them at lower power levels and all the switches seem to burn at 450 v, I don't know what to do, I've seen on uzzors design that his switches does not have internal reverse diodes, is it possible that the reverse diodes could implement a short circuit on the bridge? When the lower mosfet turns on the current runs back to the high side mosfet through the reverse diode?

previous 1 2 3 4 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob