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pulses problem of mosfet driver circuit

mshh

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I used the attached circuit to drive Buck converter but when i connect the power at the input terminal of the dc converter the pulses at the mosfet transistor gate is shifted as seen. the load is dc motor that draws almost 0.15a at 20v, switching frequency is 5khz


1606057802727.png
 

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danadakk

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What do you mean "shifted", in time, offset ....? I cannot tell where ground is from your
scope shot.

Regards, Dana.
 

mshh

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What do you mean "shifted", in time, offset ....? I cannot tell where ground is from your
scope shot.

Regards, Dana.
you can see the drop down in the pulses it should be square wave
 

KlausST

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Hi,

The bootstrap circuit needs VS to be GNDed.
This usually does the low side mosfet. But in your circuit it is missing...
Add the low side circuit and it will work.

Klaus
 

FvM

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The waveform shows that the circuit is working well.
you can see the drop down in the pulses it should be square wave
It shouldn't. In discontinuous conduction mode, the switching node shows three levels, also the gate node does. The"drop" indicates D2 conduction phase.
 

KlausST

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Hi,

I have to admit that it works without the low side switch.
In this case the inductor feedback helps to charge the bootstrap capacitor.

Klaus
 

mtwieg

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The waveform looks fairly normal for a buck converter.

The issue with using a bootstrapping driver for a buck converter like this is getting the bootstrap capacitor initially charged. If the output voltage is initially low (much lower than the driver supply voltage), then the that will allow the bootstrap cap to charge a bit when power is first applied, and you can start switching from there.

But this doesn't work in some situations, most notably when you're making a battery charger. For example, if the output is already connected to a 12V battery when the input voltage is applied, then the bootstrap capacitor will not charge. The high side driver can't bootstrap itself initially. In such cases a low side switch is necessary.
 

KlausST

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Hi,

If you did a search on other threads or the internet about driving a DC motor...then you see that
Low side control with a logic level mosfet is the most simple. No driver IC, no bootstrap circuit..

Klaus
 

mtwieg

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I changed the dc motor load to resistive load and pulses were pretty good,how to fix the dc motor load problem?
The waveforms are probably different because the load current is different. Looks like now the converter is operating in continuous conduction mode, whereas before it was in discontinuous mode.
 
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Akanimo

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Could you also post the Vds and the switch node voltage?
 

Easy peasy

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D2 needs to be fully connected - the operation in post #1 is pretty well correct - put a snubber across the mosfet and diode if you want to clean up the waveforms a bit - and/or slow the turn on of the mosfet - as the source is taken down rapidly as turn off - this capacitively couples to the gate also producing a negative dive if the impedance of the gate driver is not very low ... this is a common issue for newbies ...
 

mshh

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Could I use freewheeling diode with load to prevent spikes? Will that affect the buck converter?
 

Easy peasy

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yes but you need a resistor to separate the mosfet source from the gate driver connection
 

mshh

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The Freewheeling diode didn't help
 

Easy peasy

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where did you put it ? did you include the resistor as mentioned above? do you have CM pickup on your scope probes?
--- Updated ---

Also put a 2u2 cap film foil from drain to ckt power gnd with short leads - you may find this helps a lot ...!
 

FvM

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Do you still worry about the "spikes" observed in the post #1 waveform with low current load? If so, the problem is that you don't understand the nature of DCM (discontinuous conduction mode) which involves a three level waveform.
 

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