MP9928 : MOSFET switching problem and frequency is not constant

[D.Sanchaniya]

Hello.

I am making DC-DC buck converter using the MP9928.

Parameters:
Vin(min) = 12.5V
Vin(max) = 36V
Vout = 12V
Iout = 6A

Calculations for design:
Rfreq = 56k => fs = 350.87 kHz (Ts = 2.85 us);
L = 11.9 uH ( selected 12 uH inductor);
Rsense = 8 mohm;
Cout = 220 uF (selected from the circuit given in datasheet)

---

compensation network
{R;C} = {139.61k (calculated), 47k (soldered); 130 pf (calculated), 220pf (soldered)}
{C} = { 234 pf (calculated), 220 pf (soldered)}

I am facing some problems as per following.

1). Switching frequency is not remain constant, it varies with input voltage
2). High output ripple voltage

I simulated a circuit in MPSmart with my design data and simulation works fine. Output voltage has only 40mV ripple at 6A. Also throughout the frequency remain constant in simulation.

here, are some scope pictures for better understanding.

voltage across high side and low side switch

In a given picture switching frequency seems to be 125kHz (Ts = 8us).

Please respond me as soon as possible.

Thank you.

[jshannon]

What does the compensation node voltage look like? Is this your layout or an EVAL board, Eval boards are great tools. I would guess that the compensation needs to be adjusted.

[MPSNow_Saurabh]

Hi @D.Sanchaniya,

Can you please provide your schematic? If you’re comfortable sharing it here or you can reach out using MPS NOW Technical Support form: MPS NOW Technical Support - Contact

Thank you
Saurabh

[D.Sanchaniya]

Hello @MPSNow_Saurabh

Thank you for your response.

Schematic:

[D.Sanchaniya]

Calculation for given schematic

Thanks
Divyeshkumar Sanchaniya