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Hardware Design Guide: UC3843 Analog Controller

This document provides the exact pinout, electrical requirements, and external supporting component list for integrating the UC3843 into the 4kW MPPT Booster.

This circuit configures the UC3843 as a "Hardware Slave" to the dsPIC30F2010. The dsPIC commands the target voltage, but the UC3843 handles all the nanosecond-level switching, dead-time, and peak current protection.

1. Pin Diagram (DIP-8 / SOIC-8)

            +---+--+---+
       COMP | 1      8 | VREF (5.0V)
            |          |
 VFB (from) | 2      7 | VCC (15V)
   (dsPIC)  |          |
     ISENSE | 3      6 | OUT (to Gate Driver)
            |          |
      RT/CT | 4      5 | GND
            +----------+

2. Pin-by-Pin Wiring & External Components

Pin Name External Components Required Purpose & Connection
1 COMP 100kΩ Resistor + 10nF Capacitor in series to Pin 2 Error Amplifier Compensation. Stabilizes the internal op-amp so it doesn't oscillate when the dsPIC changes the voltage.
2 VFB RC Filter from dsPIC OC1 pin + 10kΩ Pull-Up to VREF Voltage Feedback. The dsPIC feeds a filtered DC voltage here. CRITICAL: You must add a 10kΩ pull-up to Pin 8 (VREF) for boot safety.
3 ISENSE 1kΩ + 470pF (LEB Filter) + Slope Comp Resistor Current Protection. Receives the 0-1V signal from the INA181. Shuts off the IGBT instantly if it hits 1.0V (50A).
4 RT/CT Rt = 8.6kΩ to Pin 8.
Ct = 10nF to GND.		 Oscillator (20 kHz). These two components set the main switching frequency.
5 GND None Ground. Tie to the main clean analog ground.
6 OUT 1.3kΩ Resistor to FOD3150 Pin 2 PWM Output. Drives the LED inside the isolated FOD3150. (15V output - 1.5V LED = 13.5V. 13.5V / 1.3kΩ = ~10mA).
7 VCC 10µF Electrolytic + 0.1µF Ceramic to GND Power Supply (15V). Must be stable. The UC3843 turns on at 8.4V and turns off at 7.6V (UVLO).
8 VREF 0.1µF Ceramic to GND 5.0V Reference. Internal highly accurate 5V supply used to power the Rt resistor.

3. The Complete Hardware Schematic (Modular View)

To make the layout perfectly clear for PCB design, the schematic is broken down into three functional modules.

                 +-----------------------+
                 |                       |
      (COMP) 1 --|      UC3843           |-- 8 (VREF)
                 |                       |
       (VFB) 2 --|  PWM CONTROLLER       |-- 7 (VCC)
                 |                       |
    (ISENSE) 3 --|                       |-- 6 (OUT)
                 |                       |
     (RT/CT) 4 --|                       |-- 5 (GND)
                 +-----------------------+

Module A: The MPPT Summing Node (Pin 1 & 2)

This module handles the 380V hardware regulation and the dsPIC MPPT injection logic.

                            +---> To Pin 1 (COMP)
                            |
                     [10nF + 100kΩ]
                            |
[ 380V DC Bus ]---[ 1.5MΩ ]-+---> To Pin 2 (VFB)
                            |
    [ Ground ]----[ 10kΩ ]--+
                            |
                     [47kΩ Injection]
                            |
                         (Node A) --- [ 1µF Cap ] --- GND
                            |
                     [10kΩ Filter]
                            |
   [ dsPIC OC1 ]------------+-------- [ 10kΩ Pull-Up ] --- +5V

Module B: Current Sense & Slope Compensation (Pin 3 & 4)

This module handles the nanosecond current protection and cures sub-harmonic oscillation.

[ INA181 OUT ]---[ 1kΩ ]----+-------> To Pin 3 (ISENSE)
                            |
    [ Ground ]---[ 470pF ]--+
                            |
                     [10kΩ Slope Res]
                            |
                         [Emitter]
                           2N3904 NPN
                         [Base]   [Collector] ------> To Pin 8 (VREF)
                            |
                            +-----------------------> To Pin 4 (RT/CT)
                            |
                         [Rt 8.6kΩ] ----------------> To Pin 8 (VREF)
                            |
                         [Ct 10nF ] ----------------> Ground

Module C: Power & Output (Pins 5, 6, 7, 8)

This module provides power and drives the isolated gate driver.

        [ Ground ] <------------------------- Pin 5 (GND)

[ FOD3150 Pin 2 ] <-------[ 1.3kΩ ]---------- Pin 6 (OUT)

  [ +15V Power ] ---+------------------------ Pin 7 (VCC)
                    |
      [ Ground ] ---+---[ 10µF + 0.1µF ]

      [ Ground ] ---+---[ 0.1µF ]------------ Pin 8 (VREF)

Schematic Breakdown & Final Physics:

  • Pin 1 & 2 Loop: The 10nF capacitor and 100kΩ resistor connect Pin 1 and Pin 2 together to stabilize the internal amplifier.
  • Pin 2 (The Summing Node): This relies on Kirchhoff's Current Law (KCL).
    1. 380V Hardware Regulation: A 1.5MΩ resistor connects to the 380V DC Bus, and a 10kΩ resistor connects to Ground. If the dsPIC is at 50% PWM, it becomes invisible, and this divider clamps the hardware at exactly 377V.
    2. dsPIC RC Filter: The OC1 PWM signal first passes through a 10kΩ Filter Resistor and a 1µF Filter Capacitor to Ground. This flattens the 20kHz PWM into a perfectly smooth DC voltage.
    3. dsPIC Injection: This smooth DC voltage connects to Pin 2 via a 47kΩ injection resistor. By changing PWM from 0% to 100%, the dsPIC slides the hardware target voltage anywhere between 443V and 311V.
    4. Boot-Up Safety: A 10kΩ Pull-Up resistor is attached directly to the dsPIC OC1 pin to the +5V MCU power. If the dsPIC is booting (floating), this forces 5V through the filter, regulating the bus safely to 321V and keeping the system shut down.
  • Pin 3 Input: The INA181 current signal passes through a 1kΩ resistor and 470pF capacitor to ground to filter out 100ns Leading Edge Noise.
  • Pin 3 & 4 (Slope Comp): The 2N3904 NPN transistor's Base connects to Pin 4 to read the 20kHz sawtooth wave without stealing current. Its Collector connects to Pin 8 (VREF) for muscle. Its Emitter connects through a 10kΩ slope resistor to Pin 3 to cure sub-harmonic oscillation at >50% duty cycles.
  • Pin 4 Timing: An 8.6kΩ resistor connects to Pin 8. A 10nF capacitor connects to Ground. This guarantees exactly 20kHz switching.
  • Pin 6 Output: A 1.3kΩ resistor connects directly to the LED input of the FOD3150. Since the UC3843 outputs 15V, this resistor safely limits the LED current to ~10mA.

4. The Slope Compensation Circuit (Deep Dive)

As discussed in Section 14.8, because you are boosting 150V to 380V (a duty cycle greater than 50%), you must add slope compensation to Pin 3 to prevent sub-harmonic oscillation.

How to wire it:

  1. NPN Transistor: Use a standard cheap NPN like a 2N3904.
  2. Base: Connect to Pin 4 (RT/CT). This picks up the 20kHz sawtooth wave.
  3. Collector: Connect to Pin 8 (VREF) or Pin 7 (VCC) for power.
  4. Emitter: Connect a 10kΩ resistor from the Emitter to Pin 3 (ISENSE).

How it works: It takes the oscillator's rising ramp and mixes a tiny bit of it into the current sensor's signal. This mathematically tricks the UC3843's internal comparators into remaining perfectly stable, even when the duty cycle climbs up to 90% during a cold start!

5. The "Dead-Boot" Explosion Risk (CRITICAL PCB FIX)

When reviewing Advanced Chinese inverter architectures (like Voltronic Axpert), they all implement a safety mechanism to prevent a "Dead-Boot Explosion."

The Flaw: When you turn the inverter on, the 15V supply wakes up the UC3843 instantly. However, the dsPIC30F2010 takes several milliseconds to boot up its software and output a signal on the OC1 pin. During these milliseconds, the VFB (Pin 2) is floating at 0V. If VFB is 0V, the UC3843 thinks the solar voltage has crashed, and it instantly goes to 100% MAXIMUM Duty Cycle. Turning on a boost converter at 100% duty cycle is a dead short circuit across the solar panels. The IGBTs will explode before the dsPIC even finishes booting.

The Tier-1 Solution: You MUST place a 10kΩ Pull-Up resistor on the dsPIC OC1 PWM line (connected to your 5V MCU power).

  • Why this works: Before the dsPIC boots, this resistor pulls the injection line to 5.0V. This 5V feeds through the 47kΩ injection resistor into Pin 2 (VFB), pushing the VFB node slightly above 2.5V. The UC3843 defaults to 0% Duty Cycle (safely disabled).
  • Once the dsPIC boots up, it drives the OC1 pin low or high, overpowering the pull-up resistor and taking proper control of the voltage.