I used EV174A-J-00A.
Adjust the output voltage to 15.5V with EV174A-J-00A.
I passed a load of 100mA, but the output voltage dropped to 14.88V.(15.5V⇒14.88V)
Question
How can I improve the output regulation at 100mA load currents?
Best regard.
Hi Hideyukisan:
Thank you for your post and considering the MPS174 for your application. I am assuming your total load current range is 100mA to 300mA. Please correct me if I am wrong.
The change in output voltage is due to the change in the Forward diode drop of D2 (1N4007) and D5 (STTH1R06) at 100mA and 300mA of load current. The Forward drop of D2 remains constant between 100-300mA but the diode drop of D5 changes as the load changes from 300mA to 100mA. Please try to change the diode D2 to STTH1R06 (same as D5) and then adjust the output voltage to 15.5V by changing the R3,R4 combination. Also another idea to reduce the total variation between 100-300mA is to adjust the Output voltage to 15.5V at 200mA (mid-point of the total current range) and then check the output voltage at 100mA and 300mA. Please let me know what you find.
Best regards,
Prasad
Hi Prasad,
Thank you very much for your answer.
May I ask add question?
Q1
Looking at the diode data sheet,
The forward voltage of STTH1R06 of D5 is larger than the forward voltage of 1N4007 of D2.
Should I change D2 to STTH1R06 (same as D5)?
Q2
What is the role of D2 and D5 in this circuit?
Best regard.
Hi Hideyukisan:
Answer Q1: Yes please change D2 to STTH1R06 ( same as D5)
Answer Q2: The role of D2 is to sample and hold the Output voltage when D5 is conducting. The cap C1 holds this voltage as D2 conducts the same time D5 is conducting. The role of D5 is as a freewheeling diode and the output inductor discharges into the load when the internal mosfet of MP174 is turned off. Also it clamps the pin 3 of MP174 from going below 0.7V.
Best regards,
Prasad
Hi Prasad,
Thank you very much for your answer.
I try your answer.
Best regard.
Hi Prasad,
I changed the diode and checked it,
The output voltage drop when connecting the load did not improve.
（Question）
From the measurement result,
Is the voltage drop when the load is connected a characteristic of DCDC regardless of the peripheral circuits?
Best regard.
Hi Hideyukisan:
Yes the diode drop affects the Output voltage drop as it is the freewheeling diode and is in series with feedback resistors. As a result the feedback voltage gets DC biased because of this drop. The voltage drop of the freewheeling diode changes with load current and so the feedback also changes with load current. Ideally you want to use a freewheeling diode whose ON drop does not change with Load over the entire load range, At very light load the effect of this diode is more visible or pronounced. You may also want to sue some minimum load to reduce this output voltage variation.
Hi Hideyukisan:
Can you please share the Inductor value that you have used?. It also helps to increase the inductance value so that the converter stays in CCM and the freewheeling diode is conducting at all times. This is necessary for the feedback sampling network to work well. Please also provide the Input Line voltage range for your application and also the load current range in which you want tight regulation.
Are you measuring the output voltage correctly? The negative lead of your voltmeter should be connected to the negative of the eval board, similarly the +tive lead of your voltmeter should be connected to the positive of the eval board directly. If you are sensing your voltage, remotely like say on your load board via some long thin wires to the eval board then there will be drop in the wiring, despite the fact that the voltage on the eval boar maybe well regulated.
Hi Prasad,
Hi jshannon,
Thank you for your answer.
I understood characteristic of DCDC regardless of the peripheral by Prasad reply answer.
Thank you for advice by Jshannon reply answer.
Best regard.
Thank you for using MPS Hideuki-san.