# Odd hundreds of amps when switching on braking after adding braking resistor to H-bridge

I am trying to deal with current being rejected by a H-bridge when doing dynamic braking of a motor: to control the amount of braking, the PWM duty cycle adjusts the time the motor spends coasting (all transistors off) vs being short-circuited (both high transistors on).

When coasting, the braking current still flowing goes through the H-bridge's body diodes and tries to make its way to the supply, but it is unidirectional. So I've added 3 Zeners in series to take care of transients, but I would like to redirect the bulk of the power dissipation to a power resistor, so I've added one. Without associated chopper, I don't mind the inefficiency.

After adding in the resistor, very regularly within 10ms for example, I have hundreds of amps flowing from the supply (dark blue) - and 10A through the Zeners (green). These spikes always occur when the high transistors are switched on.

Where do these spikes come from and how to remove them (a diagram would be nice if relevant)?

P.S: DC brushed Motor parameters: R=10 Ohm, L=50mH, backEMF=40V, motor current (outU to outV)=1A decreasing down to -2A in 10ms. PWM duty cycle is 50%.

Edit: This spike goes through all transistors AND diodes of the lower half to ground! What's happening?

Edit 2: Adding a 10uF capacitor in parallel of the H bridge solved it - but I am still curious about why this happened.

• Please show your input waveforms. You speak of your switching elements as transistors, but your simulation shows ideal switches. Which is it? For that matter, your sim traces have the currents labelled as Id5and Id8, which suggests that your circuit is using FETs. Please show the circuit you are actually using, or show a characterization of your switching elements. Commented May 4, 2020 at 23:02
• Where did you add the capacitor - show it on the schematic. Commented May 5, 2020 at 1:56
• If you're still curious about what happened, please show the whole circuit. It is now missing the connection to outU and outV at the bottom, as well the model cards (.model statements) Commented May 6, 2020 at 9:18

You state the differential load (ie the motor) has a resistance of 10R and equally you have a DClink voltage of 30V.

If you were to permanently turn on a diagonal pair of switches you would only draw 10A from the supply.

Now if you were to turn both switches on in a leg (bad control or no dead-time added to mitigate switching times), then you would present the battery with a "short-circuit" and thus a significant amount of current could flow.

Plt the drive signals when such current spikes occur, I wouldn't be surprised if there is some overlap

• THanks for your quick answer - unfortunately this is not possible since the low transistors are always off - in this case study I am only switching the high transistors (both at the same time). Commented May 4, 2020 at 22:41
• but are you sure there is no current flowing through the switches. Plot the current through all switches
– user16222
Commented May 4, 2020 at 23:01
• Actually you are right, the current spike the supply provides goes through the low side transistors AND diodes (reverse biased). All of them, equally... Commented May 4, 2020 at 23:03
• Updated my post Commented May 4, 2020 at 23:07
• Final update, adding a cap solved it but I still don't understand why it happened. Commented May 4, 2020 at 23:53

You need to plot the current through various paths to determine where the current is flowing.

It can't be through the resistor or motor as their resistance is too high.

The maximum current that could flow through either of those is 3A.

You say the bottom switches are always open - are you sure? It has to be flowing somewhere.

• I don't understand - both transistors and both diodes of the lower half share the current provided by the supply, but the transistors are commanded to be OFF and I know because it works without braking resistor. driveUL and driveVL are 0V. Commented May 4, 2020 at 22:59
• Updated my post with a new measurement. Really odd! Commented May 4, 2020 at 23:08
• Final update, adding a cap solved it but I still don't understand why it happened. Commented May 4, 2020 at 23:53
• Where did you put the capacitor? Commented May 5, 2020 at 1:57