Chopper Control Strategies

Chopper control strategies dictate how the average output voltage is regulated by varying the duty cycle of the power switch. These techniques are standard in DC motor drives like traction units using IGBTs.

Core Concept

Time Ratio Control regulates the output voltage by changing the ON time or the switching period. Constant frequency operation keeps the switching frequency at 20 kHz while varying the ON time. This makes filter design straightforward and predictable.

Variable frequency operation keeps the ON time constant while changing the total period. This creates a wide range of frequencies that can cause severe harmonic issues in the output filter. Modern drives rarely use this method due to acoustic noise and poor efficiency.

Current limit control turns the switch ON when load current falls to a minimum limit and OFF when it hits a maximum limit. A typical hysteresis band might operate between 10 A and 15 A. This maintains strict control over the inductor ripple current.

Key Formulas

Average output voltage Vavg = D * Vs. Duty cycle D = Ton / T. Chopping frequency f = 1 / T. Output power Pout = Vavg * Iavg. Ripple current depends on inductor value and switching frequency.

Given: Vs = 200 V, f = 10 kHz, duty cycle D = 60%, R = 10 ohm
Formula: Vavg = D * Vs, Iavg = Vavg / R, Ton = D / f
Steps:
1. Vavg = 0.6 * 200 = 120 V
2. Iavg = 120 / 10 = 12 A
3. Ton = 0.6 / 10000 = 60 microseconds
Final Answer: Vavg = 120 V, Iavg = 12 A, Ton = 60 us

Exam Tip: Understand the difference between Time Ratio Control and Current Limit Control. Constant frequency PWM makes filter design easier and is a highly tested concept in GATE.

Performance Parameters

• Switching frequency typically ranges from 10 kHz to 50 kHz for IGBTs.
• On state voltage drop is roughly 1.5 V for standard traction devices.
• Switching loss scales linearly with operating frequency.
• Ripple factor decreases proportionally at higher switching frequencies.
• Converter efficiency frequently exceeds 95 percent under full load.

Quick Revision

• Constant frequency PWM is the preferred control scheme.
• Variable frequency complicates output filter design.
• Duty cycle linearly determines the average output voltage.
• Current limit control ensures safe device operation.
• Fast devices like the IRG4BC30KD minimize switching losses.
• Exam trap: Using the variable frequency formula for a constant frequency PWM problem.