Newbie
Joined: 25/10/2020
Location: United StatesPosts: 3
| Posted: 07:57pm 27 Oct 2020 |
I too have been looking for a schematic for these Chinese boost converters, and came across this thread early in my search. Having found no schematic anywhere, I decided to make one myself and share it here for others who may be interested:
QS-4884CCCV-1800W schematic.png
They are surprisingly well made, and are capable doing 1800w if the input/output voltages are high/close enough (e.g. >48/60v; the inductor overheats otherwise). The maximum rating of the Schottky diodes combined is 40a, and the output terminals are rated at 30a. They even have ingenious reverse protection by using a mosfet backwards! They have Over Current Protection, where it throttles back if the input current goes too high (not sure how well it works though), which is separate from the Constant Current circuit.
However, they have one glaring design flaw: The blue UVP LED drops too much voltage, causing a danger zone in the 5-10v range where the converter tries to run but has too little voltage to fully drive the switching mosfet or to turn itself off, which can burn up the mosfet when under load. This isn't helped by the nearby feedback capacitor (between pins 1 & 2), which makes the UVP circuit incapable of reacting fast enough or regulating the input voltage, sending it into dangerous spasms instead. I would strongly suggest replacing the UVP LED with a red one and knocking the feedback capacitor off the circuit board as a fundamental design modification before using these converters. The feedback capacitor can be replaced with a 10-100k resistor to reduce noise and minor oscillations when regulating the input voltage if desired.
Another couple shortcomings is the lack of Over Temperature Protection, and a fan that has two speeds (on and off) with a tendency to vibrate (where the axle starts oscillating in the bearing, dragging the impeller RPM way down and reducing cooling).
I am successfully using these boost converters as mini-MPPTs in a partially/occasionally shaded 36v solar system, specced at 1000w/board. Loaded panel nominal is 30-35v and battery float is 43v, so the boost converters do a decent job with UVP set to 32v and the modification above in place so it can function correctly, regulating the input voltage. The advantage this setup has over a traditional, high voltage MPPT system is that partial shading over one panel doesn't take out an entire series array section.
As I've watched the system function and adjusted the parameters manually, I realize there's a bit more power to squeeze out in full noon sunlight (panels work better at 35v), and in heavy clouds (panels work better at 30v), and at dusk (panels drop below 30v and work best anywhere from 15-25v). Hence the need for a schematic so I can add basic (passive) Power Point Tracking, Variable Fan Speed, and Over Temperature Protection. The modifications are quite simple:
1: Remove the 22K CC pullup resistor (between TL494 pin 15 and 5v). This leaves the CC circuit hanging at minimum power (~250mA out), regardless of the CC adjustment.
2: On the underside of the circuit board, connect a 30v zener diode (adjust according to the start of your power point curve) from Vin to a 10k resistor connected to the non-grounded lead of the CC potentiometer. This will cause the CC adjustment to rise starting at the input voltage set by the zener diode.
3: Remove all the fan control components except for the PNP transistor and its 4.3k pulldown resistor.
4: Bridge the two pads behind/parallel with the 4.3k pulldown resistor. This should connect the left lead of the thermistor to the transistor's base.
5: Bridge the two pads between the two TL431s, where the 10k resistor used to be. This should connect the right lead of the thermistor to ground. Now the fan speed will gradually increase from stopped as the heatsink warms up. Just be careful not to short the thermistor during operation or you will blow the transistor as there's no series base resistor (by design; we need the highest sensitivity we can get so the temperature curve isn't too wide). The temperature threshold can be adjusted by changing the value of the 4.3k pulldown resistor.
6: Now that the fan speed is variable, we can add OTP. Solder a 10v zener diode from the positive fan lead to the top lead of the UVP potentiometer (should be connected to LM358 pin 3). As the fan approaches full speed, the zener will start conducting, rapidly raising the UVP point, causing the UVP LED to light and the boost converter to throttle back or even stop completely until the heatsink cools down a little.
7: Another change I am contemplating is to break the signal ground trace and connect signal ground directly to main ground. The rationale is that the solar power point is in relation to input current, not output current. Additionally, the CC circuit can't go any lower than 250mA (out), but testing has demonstrated that it can go down to about 100mA with the main ground, which eliminates the ground loop caused by the 100uF 12v filter capacitor, and includes the input current (as measured by the circuit board traces) in the CC measurements. This may provide better power point tracking performance in the heavy clouds to dusk range.
Regarding these boost converters being used as MPPTs, there's a guy on Reddit who actually replaced the TL494 switching logic with an ESP32, resulting in active power point tracking, WiFi connectivity, and a nice, visual web interface!