Wuxi Jinle Automobile Motor Factory , https://www.wxjldj.com
Basic knowledge of circuit design: modulation method of digital power supply
Digital power modulation techniques are primarily categorized into Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM). These methods control the energy transfer in power systems by adjusting either the width or frequency of the switching pulses.
**1. Pulse Width Modulation (PWM)**
PWM is a widely used technique where the switching time of the power transistor is adjusted by varying the duty cycle of the pulse, while keeping the switching frequency constant. This method is often referred to as "fixed frequency adjustment." In contrast, PFM adjusts the switching frequency to control the on-off ratio of the power switch, without changing the pulse width, which is known as "fixed-width modulation."
PWM can be further divided into two main categories: fixed frequency control and variable frequency control. Variable frequency control includes techniques such as constant hysteresis loop width control, fixed on-time control, and fixed off-time control.
- **(1) Constant Hysteresis Loop Width Control**
This method uses a Schmitt trigger to control the switching. When the output voltage reaches a certain threshold, the Schmitt trigger switches state, turning the power transistor off. Once the voltage drops below a lower threshold, the transistor turns back on, repeating the cycle.
- **(2) Fixed On-Time Control**
A monostable trigger is used to set a fixed on-time for the power transistor. After the on-time elapses, the transistor turns off automatically, allowing the system to reset and repeat the cycle.
- **(3) Fixed Off-Time Control**
Similar to fixed on-time control, this method sets a fixed off-time using a monostable trigger. The transistor turns on after the off-time has passed, ensuring consistent timing between cycles.
These methods fall under frequency-variable control, which simplifies circuit design but introduces challenges in managing noise and electromagnetic interference due to the non-fixed switching frequency.
**(4) Fixed Frequency Control**
Fixed-frequency control is currently the most commonly used approach. It offers several advantages, including easier design of transformers and filters, which helps reduce electromagnetic interference. Additionally, it is easier to source high-performance and cost-effective PWM control chips.
In fixed-frequency control, the system uses a clock signal to set the switching frequency. An error amplifier compares the output voltage with a reference, generating an error signal. A sawtooth wave is then compared with this error signal to determine when to turn the power transistor on and off, maintaining stable operation.
The key components in a fixed-frequency control system include a clock generator, an error amplifier, a comparator, and a ramp signal (such as a sawtooth wave).
**2. Pulse Frequency Modulation (PFM)**
PFM adjusts the switching frequency based on the load conditions rather than the pulse width. This allows the system to operate at different frequencies depending on the input voltage and load demand.
A typical PFM cycle consists of three phases: inductor current rising, inductor current falling, and inductor current remaining zero. Two main mechanisms are used to manage these phases:
- **First Mechanism**: The frequency and duty cycle of the inductor current remain constant. As the load decreases, the "idle" period increases, reducing the overall operating frequency.
- **Second Mechanism**: A monostable flip-flop is used instead of an oscillator. It works in conjunction with a current-sensing circuit to control the power transistor's on and off states, and it triggers the idle phase when necessary.
PFM includes several variations, such as Clock-Simulated PFM, Adjustable Period PFM, and Current-Limited PFM. These methods share similar principles, with one common example being the Clock-Simulated PFM.
In this mode, the system continuously monitors the output voltage. If the feedback voltage is below a reference value, the system enters an active state, charging the inductor. If the voltage exceeds the reference, the system enters an idle state, cutting off the power transistor until the voltage drops again. This dynamic adjustment of active and idle periods determines the effective switching frequency, making it ideal for low-load applications.
Hydraulic AC motor refers to the AC motor used in the hydraulic system, mainly used to drive hydraulic pumps or other executive components. The AC motor uses alternating current as the power source, and has the advantages of simple structure, low manufacturing cost and long service life .