Theory of Operation

Theory of Operation

Circuit:UC2524 Soft-Start

Written:Bryce Salmi

UC2524 Soft-Start Overview

Fox-2 will be launched into a 600-800 km sun-synchronous orbit with passive magnetic orientation. The satellite will spin at several revolutions per minute around its own axis as it orbits around the Earth. As the spacecraft rotates, each MPPT will turn on and off as its respective panel points towards the Sun and the black of space. If the MPPT DC/DC switching circuitry were to turn on as fast as possible, several issues could arise such as ElectroMagnetic Interference (EMI) and excessive start-up current to charge the input and output capacitances. Our customer, The Radio Amateur Satellite Corporation (AMSAT), has specified that the maximum power point is to be achieved within 100 ms of Sun acquisition. This circuit delays the start-up of the UC2524 for about 25 ms to 50 ms, providing ample time to find the maximum power point but also limit start-up current.

Circuit Operation

The UC2524 internal error amplifier output is connected to the compensation pin providing access used to directly control the PWM duty cycle. The UC2524 also powers itself with the internal 5V reference guaranteeing that if the reference is not present, the device will be off. The soft-start circuit in Figure 1 is able to sink the error amplifier output current to ground through the compensation pin and effectively force a 0% duty cycle during start-up. Op-amp U1 represents the UC2524 error amplifier in which the output will be pulled to ground by Q1, a PNP bipolar transistor. When the solar panel is in darkness, the UC2524 it powers is off and the PNP transistor is in saturation, pulling the compensation pin down to ground. When the circuit starts up, the base of Q1 is connected to the UC2524 5V reference through R1 and is in parallel with capacitor C1, forcing a low voltage on the base until the capacitor is charged. The 5V reference turn on event is simulated with a step voltage from the voltage supply V1 resulting in C1 charging up to 5V over time. As C1 charges, Q1 slowly turns off. When C1 has charged to 5V, Q1 will be off indefinitely until the solar panel rotates back into darkness.

The RC circuit turn on characteristics can be modeled with the following equations:

(1)

Where is the RC time constant of R1 and C1 andis the fundamental frequency of the RC pair.

(2)

Where is the rise time of the voltage on the capacitor.

R1 is 49.9kΩ and C1 is 0.47μF, producing the RC time constant shown in Equation 1 which can be solved for the fundamental frequency,. Using the fundamental frequency, one can also calculate the rise-time of the soft-start circuit with Equation 2. Due to extra base current of Q1 into C1 from the forward-biased base-emitter junction, the capacitor charges up quicker than predicted with the RC circuit. This causes a small inaccuracy on the calculated rise time seen in Figure 2 that cannot be easily predicted. The calculated fundamental frequency and rise time are shown below.

1