The SDH8594AS is a high-precision primary-side control switching power supply chip with an integrated high-voltage MOSFET. It is an offline switching power supply integrated circuit, featuring external line loss compensation and built-in peak current compensation. By detecting the peak current of the transformer's primary winding and the response voltage of the auxiliary winding, it controls the system's output voltage and current to achieve constant voltage or constant current output. The SDH8594AS from Liwei Electronics integrates multiple protections, effectively reducing the number and size of additional components.
1. Circuit Start-up and Undervoltage Lockout: When the system powers on, the circuit charges the external capacitor on the vCC pin using the built-in depletion-type MOSFET. When VCC rises to 18V, the circuit starts operating and the depletion-type MOSFET is turned off. During normal circuit operation, the auxiliary coil supplies power to maintain the VCC voltage. When VCC drops to 85V and enters undervoltage lockout, the depletion-type MOSFET turns on, supplying power to the VCC capacitor. When VCC rises to 18V, the circuit starts operating again.
2. Peak current detection and LEB: When the drive is high, the MOSFET is turned on. The current of the primary coil, which increases linearly, is detected by the sampling resistor. When the set current constraint value, i.e. the peak current, is reached, the MOSFET is turned off.
3. CV Control Mode: When the MOSFET is turned off, the response voltage is positive. Sampling is performed between 2/3 and 1/2 of the positive FB time point. The sampled voltage is compared with the constant voltage threshold Vc and amplified to generate the turn-off time ToF of the constant voltage loop, thus achieving constant output voltage. Under light or medium load conditions, the constant voltage loop generates different peak currents.
4. CC control mode: The circuit calculates the time when FB is positive, negative or quasi-resonant. The time when FB is positive is ToF1, indicating that there is current in the secondary coil of the transformer. The time when FB is negative is TN. The time when FB oscillates at a decaying rate is ToFF2. During these two times, there is no current in the secondary coil of the transformer.
5. Peak Current Compensation: The peak current value detected in practice due to the turn-off delay increases with the increase of the input AC voltage. Since the peak current value directly reflects the output current, the linear regulation of the output current with the input AC voltage is relatively poor. The sDH8594AS uses the turn-on time to control the peak current, so that the peak current remains essentially constant under different input voltages, thus improving the regulation of the output current.
6. Line Loss Compensation: In practical applications, the output voltage will experience varying degrees of voltage drop VAB on the cable. Under different current conditions, the voltage drop VD of the rectifier diode at the output terminal will also change, requiring comprehensive consideration. The SDH8594AS uses an internal current source to generate a offset-dependent voltage VFB. The magnitude of the internal current source is proportional to the magnitude of the output current. When the load current changes from full load to no load, the offset voltage VFB increases, and the increase can be adjusted by the reactive resistors R1 and R2.
7. Various maintenance measures: The chip integrates multiple maintenance functions, including over-temperature protection, FB over-voltage protection, VCC over-voltage protection, output short-circuit protection, current limiting protection, and maximum conduction time protection. All of these functions are automatically recovered by the chip (except for current limiting protection). That is, after detecting an abnormal condition, the chip shuts down the output, the VCC voltage begins to drop to the undervoltage point, shuts down all internal modules, and charges the VCC terminal capacitor through the depletion-type MOS, repeating the chip startup process.
8. PFM Modulation Frequency Setting: The PFM modulation frequency range of the SDH8594AS from Liwei Electronics is determined by the on-time TN and the off-time T<sub>F</sub> controlled by the constant voltage loop. Therefore, when the off-time is the longest, T<sub>FFma</sub>, the system is in the minimum constraint frequency state, and the operating frequency is the lowest; when the off-time is the shortest, T<sub>FFmink</sub>, the system is in the highest frequency operating state, and the operating frequency reaches its highest.
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