The H-bridge can be operated with unipolar PWM or bipolar PWM. The switching signals are defined by the intersections of offset m(t) with the triangular signal uD(t). The left hand side curves show the operation of bipolar PWM. Here, signals s11 & s22 are always switched identically (likewise for s12 & s21). The output voltage u2 is switched between +U1 and -U1, and the average load voltage U2avg can be negative. 
The right hand side curves show the operation of unipolar PWM. Here, a second offset signal m(t) is necessary for defining the switching pattern. For s11 = on and s22 = off, there is (not shown) s12 = Not(s11) = off and s21 = Not(s22) = on. In this case the current flows through the two upper switching cells s11 & s12 (or the two lower cells), and the load voltage is zero. With unipolar PWM three voltage levels (+U1,0,-U1) are available, and the load voltage ripple is therefore reduced by a factor of 2. Also, with the PWM shown here, the pulse frequency of the voltage u2 is doubled resulting in a lower required filtering effort.

• Click onto the left hand side curves to activate the animation for bipolar PWM.
• Drag the red horizontal line of the offset m(t) to change the switching pattern and load voltage u2.
• Drag the red time slider in the curve diagram to visualize which switching cells (transistor plus anti-parallel diode) are in the on-state (green colour).
• Click onto the right hand side curves to activate the animation for unipolar PWM. Change offset signal m(t) and drag the time slider to visualize the switching cell states.