I can't understand how you are getting 2 different currents from a constant current converter.

(without changing the settings of the converter)

The output of the buck converter goes to the load through the MOSFET switching module. On the high side there is switching taking place. I connect a resistor in parallel to the switch. When constant current is set, current flows through the MOSFET when PWM is ON. When PWM is OFF, the switch opens and current flows through the resistor. Of course there is power wasted at the resistor when doing this but this is the only simple way to get that waveform.

 

Good Day I know it's been a while. I wanted to try out the below switching circuit for a supply voltage of 60 Volts DC, instead of 12 Volts DC.



I replaced the STD46P4LLF6 MOSFET with a SL50P06D MOSFET that is rated for 60 Volts and the 2N3904 with a BC 639 with a higher voltage rating as shown below.

When I tried out the circuit, I give the PWM voltage there is no switching taking place.

What am I doing wrong here. Could someone please help. Thanks.

 

HI Sab
Reduce R1 to say 2K
E
Measure Q1 Vbe

 

What am I doing wrong here. Could someone please help. Thanks.

Probably damaged the mosfet with 60 volts across the gate. Need to add R4 to reduce the gate voltage to appx 10 volts with a 60 volt supply which is also the absolute max voltage of the SL50P06. Recommend a higher voltage rated mosfet.

 

HI Sab
Reduce R1 to say 2K
E
Measure Q1 Vbe

I did it and Q1 Vbe is 2.4 Volts when PWM high is 3.1 Volts

 

Probably damaged the mosfet with 60 volts across the gate. Need to add R4 to reduce the gate voltage to appx 10 volts with a 60 volt supply which is also the absolute max voltage of the SL50P06. Recommend a higher voltage rated mosfet.

Thanks.

I will try it out. Hope the mosfet is not damaged. I will check it. The max voltage I tested it was 50 Volts not above that. I checked continuity at the MOSFET terminals, they weren't shorted. I will install the resistor as per the circuit and post the findings.

 

hi sab,
Q1 V Base to Emitter voltage should not exceed +0.7V.
Where are you measuring?
Note 'sgh' post ref VGate/Source
E

 

I did it and Q1 Vbe is 2.4 Volts when PWM high is 3.1 Volts

 

My LTspice sim indicated rather slow switching speed for the MOSFTET.
For fast switching speed to minimize MOSFET dissipation with PWM, I recommend changing (EDIT) R1 R2 to 1kΩ and R4 to 5.1kΩ(1W).

 

R4 to 5.1kΩ(1W).

That would make the FET's Vsg about 40V if the TS wants to use a 60V supply!

 

That would make the FET's Vsg about 40V if the TS wants to use a 60V supply!

My mistake.
I meant change R2 to 1kΩ.

 

My mistake.
I meant change R2 to 1kΩ.

As sghioto pointed out possibly the MOSFET got damaged. I will try out the resistances as specified.

For now the Vbe is 0 Volts and voltage is present at the output terminals of the circuit when the supply is connected to the input terminals regardless of PWM input. But when load is connected current does not flow.

I am thinking of ordering higher voltage rated MOSFET.

These are the P Channel Power MOSFETs available. Could you please suggest which of the below MOSFETs have a higher switching speed and best suited...

1. RD3N01BATTL1-ROHM-Power MOSFET, P Channel, 80 V, 10 A

2. SQD50P08-25L_GE3-Power MOSFET, P Channel, 80 V, 50 A, 0.02 ohm, TO-252AA,

3. IRF9520SPBF-Power MOSFET, P Channel, 100 V, 6.8 A, 0.6 ohm, TO-263AB

Thanks.

 

I would suggest the one with the lowest resistance. #2.

 

Does the PWM from the micro need to be isolated?
Version #1:
View attachment 362423

AND, now I see thatI have missed a lot of coments. Sorry about that. Non-applicable comment deleted.

 

As sghioto pointed out possibly the MOSFET got damaged. I will try out the resistances as specified.

There is almost no possibility that the MOSFET survived -60v Vgs as sghioto suggested.
Using a simple resistor divider won't work if you're switching above 1kHz the MOSFET’s gate capacitance will turn your square wave into a ramp. This causes the FET to linger in the linear region, leading to overheating and if pushed to its max current, will certainly die. To drive a P-channel MOSFET at high speed without messing up your duty cycle, you need to use a totem-pole topology (I post below) or if you want to be really pro, you need a gate driver chip. The part numbers I used were just because they were close to your design, but not necessarily have to be. Just the resistors matter and the 11v Zener diode. I made a Spice and seems to be working? The output is crisp square at 50kHz and the Vgs is -11v which is standard

 

The above post wouldn't apply if the PWM signal is at .1hz, 5 seconds ON and 5 seconds OFF.
Unless I'm mistaken about that.

 

There is almost no possibility that the MOSFET survived -60v Vgs as sghioto suggested.

To drive a P-channel MOSFET at high speed without messing up your duty cycle, you need to use a totem-pole topology (I post below) or if you want to be really pro, you need a gate driver chip. The part numbers I used were just because they were close to your design, but not necessarily have to be. Just the resistors matter and the 11v Zener diode. I made a Spice and seems to be working? The output is crisp square at 50kHz and the Vgs is -11v which is standard

May be I am not able to read this circuit properly...

Where is the input +60 Volts supply in this circuit....

 

The above post wouldn't apply if the PWM signal is at .1hz, 5 seconds ON and 5 seconds OFF.
Unless I'm mistaken about that.

I am using upto 2 kHz. That 0.1 Hz I used for demonstration purpose...

 

May be I am not able to read this circuit properly...

Where is the input +60 Volts supply in this circuit....

View attachment 364175

V2

 

V2

And where is the Load output (pulsed). I am new to simulation circuits so I need some help reading them. Thanks.