# Difference between Time Ratio Control and Current Limit Control

The industrial applications require power from the resources of DC voltage. Many of these applications, but, achieve better in case these are fed from adaptable DC voltage sources. The alteration of fixed DC voltage to a variable DC o/p voltage, the utilization of semiconductor devices are termed as chopping. A chopper is a fixed device, used to convert static DC i/p voltage to a variable o/p voltage straight. It is a high-speed ON/OFF semiconductor switch. For a chopper circuit, force commutated thyristor, GTO, power BJT, and power MOSFET are used as the power semiconductor devices. A chopper may be thought of as the DC equivalent of an AC transformer since they perform in an identical manner like a transformer. A chopper is used to step up or step down the fixed DC i/p voltage. The chopper system gives high efficiency, smooth control, regeneration, and fast response. There are two kinds of control strategies used in DC choppers namely time ratio control and current limit control.

## Time Ratio Control and Current Limit Control

There are two kinds of control strategies used in DC choppers namely time ratio control and current limit control. In all situations, the average value of the o/p voltage can be changed. The differences between these two can be discussed below.

### Time Ratio Control

In the time ratio control the value of the duty ratio, K =TON /T is changed. Here ‘K’ is called the duty cycle. There are two ways to achieve the time ration control, namely variable frequency and constant frequency operation.

#### Constant Frequency Operation

In constant frequency control strategy operation, the ON time TON is changed, keeping the frequency, i.e, f=1/T, or time period ‘T’ constant. This operation is also named as PWM (pulse width modulation control). Hence, the output voltage can be varied by varying ON time.

#### Variable Frequency Operation

In variable frequency control strategy operation, the frequency (f = 1/T) is changed, then the time period ‘T’ is also changed. This is also named as a frequency modulation control.In both cases, the o/p voltage can be changed with the change in duty ratio.

The disadvantages of Control Strategy include the following

• The frequency has to be changed over an extensive range of the o/p voltage control in FM (frequency modulation). The design of the filter for wide frequency change is quite difficult.
• For the duty cycle ration control. Frequency change would be varied. As such, there is a possibility of intrusion with systems by positive frequencies like telephone lines and signaling in frequency modulation (FM) technique.
• The huge OFF time in FM (frequency modulation) technique may make the load current irregular, which is unwanted.
• Therefore, the constant frequency system with pulse width modulation is preferred for choppers or DC-DC converters.

### Current Limit Control

In a DC to DC converter, the current value varies between the maximum as well as the minimum level of constant voltage. In this method, the DC to DC converter is turned ON & then OFF to confirm that current is preserved constantly between the upper limits and also lower limits. When the current energies beyond the extreme point, the DC-DC converter goes OFF.