Linear Shunt DC-DC Converter

The alternative for a linear series voltage converter is a linear shunt voltage converter. The principle of operation for this type of DC-DC converter is shown in Fig. 1 (a).

Fig. 1: The principle of a linear shunt voltageconverter and (b) a simple practical implementation

U_in is lowered to U_out by means of the resistive division between the fixed input resistor R_in and both the load R_L and the variable shunt resistor R_shunt , where U_out is calculated through as follows :

R_in can either be the intrinsic output resistance of  U_in, an added resistor or the combination of both.

U_out is kept constant under varying R_L and U_in conditions by adapting the value of R_shunt.

This operation can be performed by a control system, providing feedback from U_out. The control system consumes a certain amount of power by drawing a current I_cs from U_in or U_out. The fact that  U_out can also be directly used to supply the control systems is due to the self-starting nature of this circuit, as opposed to the linear series  voltage converter.

The voltage used for supplying the control system will depend on whether U_out  has a sufficiently large value. In the following analysis it is assumed that U_in is used for this purpose, for which the results merely differ little from the other possibility.

Feedback of U_out is however not always required, as illustrated by the simple practical implementation of Fig. 1(b). For this implementation the shunt resistor is replaced by a reverse-biased zener diode D. In this way a quasi constant U_out can be achieved, if the current through D is kept large enough for it to operate in the zener-region.

For a shunt converter η_lin  is calculated as below :

Fig. 2 graphically illustrates  η_lin as a function of the output power  P_out, for a constant voltage conversion ratio  k_lin.

Fig.2

The black curve is valid for the ideal case, where  I_cs is zero and the gray curve is valid for a finite non-zero I_cs. As opposed to a linear series converter,  η_lin is intrinsically linear dependent on P_out . It can be seen that  η_lin is zero for  P_out = 0 and that it has a maximal value equal to  k_lin, occurring at the maximal output power  P_{out_max} which is given below :

For a given  U_in and U_out, P_{out_max} is determined by the inverse of the value of  R_in.

Unlike a linear series converter, where η_lin is ideally independent of P_out, a linear shunt converter only achieves its maximal  η_lin at P_{out_max}. This behavior makes a linear shunt converter inferior compared to a series converter, in terms of  η_lin.

However, its simple practical implementation makes it suitable for applications that require a small and quasi constant P_out .

Furthermore, a linear shunt converter can prove to be more practical than a linear series converter in applications that have a low value for k_lin and η_lin.

In such a case the voltage difference U_in − U_out  will only be present over the passive resistor R_in rather than over an active device, of which the maximal voltage is limited.

The simple nature of a linear shunt voltage converter, and its lack of large passives, makes it suitable for monolithic integration in non-critical applications.

Obviously, the problem of on-chip power dissipation remains and becomes more limiting than for linear series voltage converters.