The control methods are important and crucial for sensitive sources and sensitive loads.
When we design a converter for a particular input/output voltages and load, whenever change in the source voltage or output reference value or load that connected to converter, we need to give exact output voltage value as per the set reference value. For this purpose we need to control the PWM pulses which are giving to the active switches used inside our converter.
Let’s understand more clearly with different scenarios
Consider one DC – DC converter (PSFB) with following specifications
- Input Voltage: 100 V – 200 V,
- Output Voltage: 10 V – 14 V,
- Power: 100 W,
- Frequency: 20 kHz
Design points: 150 V input, 12 V output, 100 W, 20 kHz
Scenario -1: The input voltage changed from 150 V to 120 V and required output voltage is 12 V
When input voltage is 150 V –> output voltage is 12 V
When input voltage is changed to 120 V –> output voltage automatically reduces from 12 V to 9 V (assume)
But reference value is 12 V –> need to do some adjustments inside the converter –> transformer turns ratio is already fixed and built –> can not change in hardware
The PWM pulses going to active switch can adjust through software programming –> to bring back output voltage from 9 V to required 12 V
How we are going to adjust PWM pulses –> depends on type of control is using
Scenario -2: The input voltage changed from 150 V to 180 V and required output voltage is 10 V
When input voltage is 150 V –> output voltage is 12 V
When input voltage is changed to 180 V –> output voltage automatically increases from 12 V to 15 V (assume)
But reference value is 10 V –> need to do some adjustments inside the converter –> transformer turns ratio is already fixed and built –> can not change in hardware
The PWM pulses going to active switch can adjust through software programming –> to bring output voltage from 15 V to required 10 V
How we are going to adjust PWM pulses –> depends on type of control is using
Types of Control:
Type of control nothing but how we are adjusting the PWM pulses which are giving to active switches placed inside the converter/system.
- Variable Frequency Control
- Fixed Frequency Control
- Asymmetrical Duty Cycle (ADC) Control
- Phase-shift control (PS) Control
Variable Frequency Control
Variable frequency control, as name saying, this type of control keep on change the frequency for controlling the converter output voltage.
Vo = Function (frequency)
In this control, the PWM pulse frequency either increases or decreases to get the required output voltage after the design as per reference value given.
Fixed Frequency Control
Fixed frequency control, as name saying, this type of control keep frequency constant for controlling the converter output voltage.
Vo = Function (duty)
In this control, the PWM pulse duty cycle either increases or decreases to get the required output voltage after the design as per reference value given.
Asymmetrical duty cycle (ADC) control
In this control, the duty cycle varies by increasing or decreasing the turn-on time of pulse with fixed frequency.
duty = Function (tun-on time)
Phase-shift (PS) control
In this control, the duty cycle varies by phase-shifting the PWM pulses respect to each-other with fixed frequency and turn-on time of each pulse.
duty = Function (phase-shift)
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