H-bridge/buck converter with negative and positive voltage range

Question

as already questioned here, i am still working on peltier controllers. For that I am using a stepper motor driver (h-bridge) with a low-pass LC filter and the peltier element behind, as Olin Lathrop kindly explained. It works like a charm and is great for what it is designed for.

The schematic for that (I will add that to my older post as well):

My problem is controlling temperatures close to the room temperature. By now, I have been using the h-bridge as a MOSFET only. That means I need to decide if the Peltier is going to cool my application, or heat it up. Since the h-bridge is capable of reversing the current I am trying to design a circuit that can go in both directions. The capacitor C1 is an electrolytic capacitor, so negative voltages would blow everything. I thought of using diodes somehow to detect current direction, but I am not really getting it. Can anyone help?

Answer 1

I think you missed the point of the diode in Olin's original schematic. It was there to protect the bare MOSFET from the flyback effects of the inductor, and provide a path for the current to flow when the inductor is discharging its stored energy. To do that with an H bridge, you would connect diodes to the supply rails, but that is sometimes handled inside the bridge chip itself. Connecting a diode directly between the H bridge outputs is pointless — and potentially dangerous, as Andy pointed out.

^{simulate this circuit – Schematic created using CircuitLab}

If you want to use large polarized capacitors in your filter, you just need to connect their negative terminals to ground. Each one will see only the correct polarity.

^{simulate this circuit}

Answer 2

Firstly, what you have drawn does not constitute a H bridge - you have shown a ground connection on the "-" end of the peltier - this is a non-starter with H bridges. Neither will a H bridge driving a peltier need a diode (D1). In fact D1 will fry the millisecond the H bridge reverses.

So....

Use a standard H bridge that is driven from a high frequency PWM signal (not unheard of) and the smoothing components (L and C) need not be that big and the capacitor need not be a polarized type at all. Anything from 10 kHz upwards would probably do the job and many H bridge circuits work in excess of 100 kHz.

Here is a product that you can buy that might give you some ideas. It's called FTX300 and it's a H bridge that works with PWM and controls a peltier of up to 180 watts.

Here is also a Texas Instruments white paper that will help you design one too.