Single-ended primary-inductor converter

A SEPIC (single ended primary inductor converter) is a DC-DC converter which allows the output voltage to be greater than, less than, or equal to the input voltage. The output voltage of the SEPIC is controlled by the duty cycle of the control transistor. The largest advantage of a SEPIC over the buck-boost converter is a non-inverted output (positive voltage). SEPICs are useful in applications where the battery voltage can be above and below the regulator output voltage. For example, a single lithium ion battery typically has an output voltage ranging from 4.2 volts to 3 volts. If the load requires 3.3 Volts, then the SEPIC would be effective since the battery voltage can be both above and below the regulator output voltage. Other advantages of SEPICs are input/output isolation provided by C1 and true shutdown mode: when the switch is turned off output drops to 0 V.

Circuit operation

The schematic diagram for a basic SEPIC is shown in Figure 1. As with other switched mode power supplies (specifically DC-to-DC converters), the SEPIC exchanges energy between the capacitors and inductors in order to convert one level of electrical potential energy (voltage) to another. The amount of power exchanged is controlled by switch S1, which is typically a transistor such as a MOSFET; MOSFETs offer much higher input impedance and lower voltage drop than bipolar junction transistors (BJTs), and do not require biasing resistors (as MOSFET switching is controlled by differences in voltage rather than differences in current, as with BJTs).

Continuous mode

A SEPIC is said to be in continuous-conduction mode ("continuous mode") if the current through the inductor L1 never falls to zero. During a SEPIC's steady-state operation, the average voltage across capacitor C1 (V_C1) is equal to the input voltage (V_in). Because capacitor C1 blocks direct current (DC), the average current across it (I_C1) is zero, making inductor L2 the only source of load current. Therefore, the average current through inductor L2 (I_L2) is the same as the average load current and hence independent of the input voltage.

Looking at average voltages, the following can be written:

$V_{IN}=V_{L1}+V_{C1}+V_{L2}$

Because the average voltage of V_C1 is equal to V_IN, V_L1 = −V_L2. For this reason, the two inductors can be wound on the same core. Since the voltages are the same in magnitude, their effects of the mutual inductance will be zero, assuming the polarity of the windings is correct. Also, since the voltages are the same in magnitude, the ripple currents from the two inductors will be equal in magnitude.

The average currents can be summed as follows:

$I_{D1}=I_{L1}-I_{L2}$

When switch S1 is turned on, current I_L1 increases and the current I_L2 increases in the negative direction. (Mathematically, it decreases due to arrow direction.) The energy to increase the current I_L1 comes from the input source. Since S1 is a short while closed, and the instantaneous voltage V_C1 is approximately V_IN, the voltage V_L2 is approximately −V_IN. Therefore, the capacitor C1 supplies the energy to increase the magnitude of the current in I_L2 and thus increase the energy stored in L2. The easiest way to visualize this is to consider the bias voltages of the circuit in a d.c. state, then close S1.

With S1 closed current increases through L1 (green) and C1 discharges increasing current in L2 (red)

When switch S1 is turned off, the current I_C1 becomes the same as the current I_L1, since inductors do not allow instantaneous changes in current. The current I_L2 will continue in the negative direction, in fact it never reverses direction. It can be seen from the diagram that a negative I_L2 will add to the current I_L1 to increase the current delivered to the load. Using Kirchoff's Current Law, it can be shown that I_D1 = I_C1 - I_L2. It can then be concluded, that while S1 is off, power is delivered to the load from both L2 and L1. C1, however is being charged by L1 during this off cycle, and will in turn recharge L2 during the on cycle.

With S1 open current through L1 (green) and current through L2 (red) produce current through the load

Note that the voltage on C1 reverses direction every cycle and it should not be a polarized type. (Please confirm, as the National App note 1484 says a tantalum or electrolytic can be used [both polarized]. Also, the voltage will not change on C1 unless the switch is closed long enough for a half cycle of resonance with L2, and by this time the current in L1 could be quite large.)

The capacitor C_IN is required to reduce the effects of the parasitic inductance and internal resistance of the power supply. The boost/buck capabilities of the SEPIC are possible because of capacitor C1 and inductor L2. Inductor L1 and switch S1 create a standard boost converter, which generate a voltage (V_S1) that is higher than V_IN, whose magnitude is determined by the duty cycle of the switch S1. Since the average voltage across C1 is V_IN, the output voltage (V_O) is V_S1 - V_IN. If V_S1 is less than double V_IN, then the output voltage will be less than the input voltage. If V_S1 is greater than double V_IN, then the output voltage will be greater than the input voltage.

Discontinuous mode

A SEPIC is said to be in discontinuous-conduction mode ("discontinuous mode") if the current through the inductor L1 is allowed to fall to zero.

Reliability and Efficiency

The voltage drop and switching time of diode D1 is critical to a SEPIC's reliability and efficiency. The diode's switching time needs to be extremely fast in order to not generate high voltage spikes across the inductors, which could cause damage to components. Fast conventional diodes or Schottky diodes may be used.

The resistances in the inductors and the capacitors can also have large effects on the converter efficiency and ripple. Inductors with lower series resistance allow less energy to be dissipated as heat, resulting in greater efficiency (a larger portion of the input power being transferred to the load). Capacitors with low equivalent series resistance (ESR) should also be used for C1 and C2 to minimize ripple and prevent heat build-up, especially in C1 where the current is changing direction frequently.

External links

Excellent Design Guidelines from National Semiconductor in Application Note 1484

References

Maniktala,Sanjaya. Switching Power Supply Design & Optimization, McGraw-Hill, New York 2005

SEPIC Equations and Component Ratings, Maxim Integrated Products. Appnote 1051, 2005.