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I am looking for a way to be able to power the H-brirdge from a boost converter.

I have a voltage source of 13V to 15V and would like to boost that up to around 17V (accuracy isnt that important) at around 4A, to power only the H-bridge.

I currently have the following circuit (please note that there are MOSFET dirvers missing):

schematic

simulate this circuit – Schematic created using CircuitLab

If I simulate the following circuit in LTspice I get the following waveforms:

output

Green line represents the voltage coming from the boost converter and the blue line represents the current through R1.

output

Here, blue line represents the current through R1, while red (M2, M5) and light blue (M3, M4) lines represent H-bridge switching. There is 1\$\mu s\$ dead-time between H-bridge switching.

Can someone please explaing what is going on? And what do I need to do to make this work as intended?

edit:

If I get rid of the H-bridge and connect R1 directly to the output of the boost converter I get the deisred result (but I need H-bridge).

R1 represents a speaker.

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  • \$\begingroup\$ How did you intend it to work? \$\endgroup\$
    – Andy aka
    Commented Mar 13, 2015 at 14:20
  • \$\begingroup\$ @Andyaka Get a flat ~17V across R1 at all times. \$\endgroup\$
    – Golaž
    Commented Mar 13, 2015 at 14:21
  • \$\begingroup\$ But the H bridge is driven at 10kHz so that cannot happen. \$\endgroup\$
    – Andy aka
    Commented Mar 13, 2015 at 14:41
  • \$\begingroup\$ @Andyaka Yes I meant with minimum '0V times'. But anyway I get these weird triangle like waveforms from the boost converter and in the current through R1. \$\endgroup\$
    – Golaž
    Commented Mar 13, 2015 at 14:45
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    \$\begingroup\$ 40kHz is sufficient, but you'll need a digital compensator. Boost converters have a right half plane zero to watch for, and digital control has transport lag, but you might be able to get your loop closed at 3-4kHz if your micro is capable of the compensation calculations between A/D samples. It's not trivial to get a good response unless you're familiar with DSP and control theory. \$\endgroup\$
    – John D
    Commented Mar 13, 2015 at 16:51

2 Answers 2

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I noticed that all your mosfets are N-channel.

It would be easier to use P-channel mosfets for M2 and M3, since the Vgs is more stable than for NMOS.

enter image description here

I think the behavior you're seeing in the waveforms is because Vgs is varying with the current through R1.

From http://www.learn-c.com/experiment7.htm:

enter image description here

There is a bit of a problem when a MOSFET is used for Switch A or Switch B. The Drain is connected to the V+ for the motor and the Source to the motor. If the gate is then made to be 2 volts greater than the Source, the MOSFET will turn on. When that happens however, the voltage at the Source will increase until the gate is no longer 2 volts greater than the Source and the MOSFET will turn off. Another voltage source is needed that is always greater than Motor V+ by at least 2 volts.

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  • \$\begingroup\$ Actually 2V is the threshold for this part, in order to get effective enhancement you need MORE than 2V. It's 29mOhms at 2.8V and 13.5mOhms at 4.5V. \$\endgroup\$
    – John D
    Commented Mar 13, 2015 at 15:29
  • \$\begingroup\$ This helped maybe a bit, the issue still persists. Thanks anyway. \$\endgroup\$
    – Golaž
    Commented Mar 13, 2015 at 15:31
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This schematic is fundamentally wrong. Mosfet driver needs to drive gates in relation to mosfet source pin not system ground. In reality this is achieved by virtual floating ground in mosfet driver chips. Alternatively you could use 4 opto isolators but this on it own could cause problems with inadequate raise/fall times.

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