Single Phase full Bridge Inverter – RL Load

In this article, we will discuss about some basics, operation (using circuit diagram) and waveform of single-phase full bridge inverter with RL load. Here we also discuss the Comparison between half and full bridge inverters.

What is a Full Bridge Inverter?

Single Phase Full Bridge Inverter is basically a voltage source inverter and it is a topology of H-bridge inverter used for converting DC power into AC power.

In case of Single Phase Half Bridge Inverter,  we require three wire DC input supply. Where as in  Single Phase full Bridge Inverter we use two wire DC input supply, which suffices the requirement. By controlling the turn ON and turn OFF time of the thyristors output frequency can be easily controlled.

Single phase full bridge inverter circuit required more component  for conversion than that used in single phase Half bridge inverters so, the cost of the circuit get increases.

The full bridge inverter circuit basically consists of 4 feedback diodes and 4 controlled switches (like Thyristor, IGBT or MOSFET).

Read more related topics: Single phase full bridge inverter with R load

Single phase half bridge inverter with RL load

Single Phase Half Bridge Inverter R load

Single phase Half Bridge Inverter circuit basically consist of four Thyristor (T1to T4) and four diode (D1to D4) these diodes are called feedback diode. Each diode is connected in anti-parallel with each thyristor. A voltage source and a RL load.

 Load RL is connected between point A and B. Point A is always considered as +ve w.r.t point B. If the current flow in this direction we assume that current is +ve.

Similarly, if current from B to A the current is considered -ve.

Due to inductive load the output voltage waveform is just similar to that of R-load. However, the output current wave form is not similar to the output voltage waveform.

In case of RL load the output, current Io is the exponential function of time.

Operation Of Single-Phase Half Bridge Inverter

The operation of full Bridge Inverter is divided into four modes.

1.  Mode I: (t1< t < T/2)        T1, T2 on   
2.  Mode II: (T/2 < t < t2)         D3, D4 on 
3.  Mode III: (t2 < t < T)            T3, T4 on
4.  Mode IV: (0 < t < t1)            D1, D2 on    

Mode I (t1< t < T/2): In this mode, we give firing pluses to thyristor T1 and T2, they will start conduct. So current flow from Vs (supply voltage) …. T1 …. load …. T2 …. back to Vs. In this mode inductor store energy and output current increases exponentially as a function of time from zero to its positive max value (Imax). and induced voltage across inductor +V_L–.

This time output voltage is also positive because point A is positive (+ve) w.r.t point B. 

Applying KVL,

 Vs – V0 = 0

The magnitude of output voltage Vo = Vs.

At time instant T/2 the output current reaches to its maximum value and thyristor T1 will be off at this time instant due to the same polarity of voltage and current.

Mode II (T/2 < t < t2): After time instant T/2 inductor dissipated energy and we know that when inductor dissipated energy it changes its polarity. And we know that the property of inductor, inductor can’t allow sudden change in current. So, inductor slowly release its energy through the D2 diode. D2 diode is conduct this time and current follow the path, Load …… D3… supply Vs …. back to Load.

 At this time interval the energy release by inductor feedback to the supply.

At this mode output current is positive but gradually decreases from Imax to zero because of energy dissipated by the inductive load. The Output voltage is negative (-Vs) this time because point B is positive w.r.t. A.

Mode III (t2 < t < T):   At time instant t2, we give firing pulse to thyristor T3 and T4, they turned on and current flow from Vs supply …. T3 …. Load … T4 …. back to Vs. So, the direction of current is reverse because point B is positive w.r.t. A and inductor store energy in reverse direction from zero to (-Imax).

At this time output voltage across load is negative (-Vs).

Mode IV (0 < t < t1):  This time interval operation is just similar to time interval after T. In this duration, D1and D2 is on.

 At time instant T the output voltage and output current having same polarity. Therefore, T3 and T4 is turned off due to inductive load and D1and D2 turned on. And current flow from load …. D1…. Vs Supply …D2 …. back to load.

Here energy release by the inductor fad back supply voltage Vs  This time point A is positive w.r.t point B. So, output voltage is positive Vs and output current decreases exponentially from its negative max value (-Imax) to zero.

Now Draw the waveform

Difference between Half Bridge Inverter and Full Bridge Inverter

Half Bridge Inverter	Full Bridge Inverter
It consist of two thyristors and two feedback diodes.	It consists of four thyristors and four feedback diodes.
The circuit cost of half bridge inverter is less as compare to full bridge inverter circuit because it required less components.	The circuit cost of full bridge inverter is high as compare to half bridge inverter circuit because it required large no of components.
The magnitude of output voltage is half of the magnitude of input DC source.	The magnitude of load voltage is equal to the magnitude of DC input source.
Half bridge inverter use three wire DC input supply.	This drawback of half bridge inverter is overcome by full bridge inverter as it requires two wire DC source.
The output power of half bridge inverter is less than full bridge inverter.	The output power of full bridge inverter is four times that of for half bridge inverter.

Frequently asked questions (FAQ’s)