Pulse-width modulation (PWM) is an efficient method for controlling the light intensity of a lamp or the speed of a simple DC motor. It lets you take advantage of the load's inertia by switching the driving signal repeatedly off for a certain time without causing a lamp to darken or a motor to slow down. The on time, or the pulse width, simply determines the average energy of the load.
Figure 1 shows the principle operation of a PWM system. The output of a triangle waveform generator is compared with an analog input signal. As long as the triangle generator output voltage is less than the analog control signal, the comparator output is high. When that output exceeds the analog input, the comparator output turns low. Raising the analog control signal over a number of PWM cycles increases the pulse width of the PWM signal.
For simple drive applications using a single IGBT in a high- or low-side configuration, the low-cost PWM generator shown below can be designed using only a quadruple operational amplifier (op-amp) and a few external components.
For maximum dynamic range, the 5V amplifier should provide rail-to-rail input and output drive capability. The combination of amplifiers A1 and A2 build the triangle waveform generator. Its output is compared through A4 with the output of a signal-conditioning difference amplifier A3. The output of A4 provides the PWM signal and typically is fed into a gate driver circuit (to drive the gate of the IGBT).
A1 is configured as a Schmitt trigger with a hysteresis of VHYS =VOUT ⋅RHYS /RFB .
The resistor values have been chosen to yield a 4.6V hysteresis, which, biased at 2.5V, results in the upper and lower input thresholds of VIT+ =4.8V and VIT– =0.2V.
The integrator A2 has a R-C time constant of:
Figure 3 helps to clarify this equation. Basically, for a difference input of VREF –VIN , the integrator output must change from VIT+ to VIT– within half the period of the PWM frequency.
From the circuit in Figure 2, we take VREF =2.5V. Figure 3 shows these results for a down-integration cycle:
- The Schmitt trigger output equals the integrator input, with VIN =VCC =5V.
- The initial integrator output equals the upper Schmitt trigger input threshold, with VOUT0 =VIT+ =4.8V.
- The final integrator output equals the lower Schmitt trigger input threshold, with VOUT(t) =VIT– =0.2V.
- The integration time t equals half the period of the PWM frequency, with t=T/2=1/(2⋅fPWM ).
Hence, for a 5kHz PWM carrier, τ=54.36μs, and choosing a standard capacitor value of CINT =4.7nF makes RINT =11.56kΩ.
The difference amplifier A3 processes ground-referenced input signals without saturation effects. Its inverting input is connected to VREF , while the non-inverting input receives the analog control signal (VANLG ). The gain factor is reduced to RF /RG =0.8, and the output voltage is calculated with:
VO =(VANLG –VREF )⋅RF /RG +VREF
For an input range of 0-5V, the output varies from 0.5V to 4.5V. Figure 4 shows the waveforms for the circuit in Figure 2.
The above circuit is applicable to a wide range of gate drivers. The design costs less than a dollar and saves you time and the hassle of programming a microcontroller.
Please join us next month as we continue our discussion of enjoying the VU and investigate methods of detecting the actual level of the audio.
For more information, visit http://www.ti.com/iso721-ca.
— Thomas Kugelstadt is a senior systems engineer with Texas Instruments. He is responsible for defining new, high-performance analog products and developing complete system solutions for industrial interfaces with robust transient protection. He is a Graduate Engineer from the Frankfurt University of Applied Science. He may be reached at