Series full converters in rectification mode

In a series (dual-transformer) full converter, two single-phase full converters are fed from two separate secondary windings of the same transformer, and their dc outputs are connected in series so that the load voltage is the sum of the two outputs: $v_o = v_{o1} + v_{o2}$.

 Sec 1 --> [Converter 1, a1] (+)--------+
                 (-)                    |
                  |    vo1              |
                 (+)                  Load
 Sec 2 --> [Converter 2, a2]          (Io)
                 (-)   vo2              |
                  +---------------------+
 Sec 1, Sec 2: two secondaries of one transformer
 vo = vo1 + vo2

Output voltage

With a continuous (highly inductive) load current, each converter gives $\frac{2V_m}{\pi}\cos\alpha$, so

$$V_{dc} = \frac{2V_m}{\pi}\left(\cos\alpha_1 + \cos\alpha_2\right)$$

The maximum output is $\frac{4V_m}{\pi}$ (both $\alpha = 0$), i.e. twice that of one converter. Hence the scheme is used for high-voltage loads.

Operation in rectification mode

• Converter 1 is kept fully ON at $\alpha_1 = 0$ (it acts like a diode bridge).
• Converter 2's firing angle $\alpha_2$ is varied from $0$ to $\pi$.
• The output then varies from $\frac{4V_m}{\pi}$ (at $\alpha_2 = 0$) down to $0$ (at $\alpha_2 = \pi$): $V_{dc} = \frac{2V_m}{\pi}(1 + \cos\alpha_2)$.
• During the interval $\pi$ to $\pi + \alpha_2$, $v_{o1} = +|v_s|$ and $v_{o2} = -|v_s|$, so $v_o = 0$: the load current effectively freewheels and no reactive power is drawn in that interval.

(For inversion, converter 1 is held at $\alpha_1 = \pi$ and $\alpha_2$ is varied.)

Advantages

• Higher output voltage with lower device voltage rating.
• Better input power factor and lower harmonics than a single converter at the same output, because only one converter is phase-controlled at a time.