I think to choose MP28167-A buck-boost converter for my application, where I need VOUT = 12V, IOUT,max = 200mA with VIN,min = 4V and VIN,max = 14V.
But I am confused with inductor value calculation.
In the MP28167-A datasheet equation (2) estimates Lbuck inductor value in buck mode and equation (3) estimates Lboost inductor value in boost mode.
With those design parameters, considering fSW = 500kHz, 40% current ripple, 85% efficiency, I get Lbuck = 42uH and Lboost = 188uH.
Are my calculations right?
Should I consider the maximum between Lbuck and Lboost calculated values?
What is the VIN values considered for the L (uH) values reported in Table 1 for common output voltages?
With VIN = 4V the MP28167-A can still supply IOUT,max = 1A at VOUT = 12V, right?
I bought one EVL28167-A-Q-00A evaluation board and I changed the feedback resistors to have VOUT = 12V. With VIN = 4,2V and IOUT = 200mA I see high ripple on VOUT. Is the 4.7uH inductor value that causes the VOUT ripple?
Hi Alessandro,
Welcome to the MPS Forum!
Regarding your inductor calculations, according to my own calculations those values are correct. Though those values seem very large. I will note, for most applications 4.7uH would be sufficient for 500 kHz operation.
Is 200 mA the absolute max or nominal load current? I would typically advise to design for worst case scenario, say 1A peak load.
I would recommend using a 10 uH inductor before moving to the 42 uH. I would choose an inductor based on which operating mode you will be in a majority of the time. Is there a nominal operating voltage at the input?
To address your other questions:
The calculations use Vin = 12V with dIL = 40% of load current assuming max 3A.
Yes, the converter should still be able to operate with Vin = 4V, Vout = 12V @ 1A as its well within operating range.
There are a few factors that can cause a high ripple at Vout. The inductor is one key player in the output ripple. To reduce Vout ripple you can increase the output capacitance or increase the inductor size, try 10 uH.
One thing I would like to point out, when operating with this buck-boost part, consider the following minimum on/off times and duty cycles for the converter given the wide range of Vin and Vout.
Thank you, Stephen.
In my application 200mA should be the absolute max load current with a nominal VIN = 12V.
I will try first to increase the output capacitance.
How should I consider the on/off times and duty cycle given my VIN range and VOUT = 12V ?
I added 330uF 35V electrolytic capacitor as C2 on my EVL28167-A-Q-00A evaluation board; the 4.7uH inductor is still present.
With IOUT = 200mA, when VIN decreases under 4.3V, now I see much less ripple on VOUT then before, but VOUT decrease from 12V to 7V about. Then VIN must increase over 6.3V to have VOUT = 12V again.
If I start with VIN = 4V, VOUT = 5.9V with a little ripple. Again, I have to increase VIN over 6.5V to have VOUT = 12V.
I think L = 10uH is needed at least, right?
What about your MPL-AL6060-150 inductor? Could L = 15uH be suitable to have VOUT = 12V @ IOUT = 200mA with VIN = 4V?
For example, operating at Vin = 4V, Vout = 12V, Iout = 200 mA the duty cycle for a boost is given by 1-(Vo/Vin). So duty cycle will be about 75%. This is more relevant when working with voltages near the output voltage.
Larger values of inductor current ripple allow the use of lower inductance, but results in
higher output voltage ripple and greater core and power device conduction losses. A smaller inductor value will store energy faster and result in a higher inductor ripple. Since the output current is so low, the FET’s will be switching for a shorter amount of time, which can result in a lower output voltage.
Increasing the inductor will slow the rate at which energy stored, which can reduce the current ripple. Try 10 uH first and check the response, if more reduction is needed you can try the 15uH. The MPL-AL6060-150 may be suitable for this.