SEPIC converter

SEPIC 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 basic schematic for a SEPIC is shown in Figure 1. As with other Switched mode power supplies, the SEPIC exchanges energy between the capacitors and inductors in order to change energy from one voltage to another. The amount of energy exchanged is controlled by S1, which is typically a MOSFET. MOSFETs are used instead of BJTs due to the extremely high input impedance and the low voltage drop across the MOSFET when turned on.

Continuous mode

A SEPIC is in continuous mode if the current through the inductor L1 never falls to zero. In a SEPIC, during steady-state operation, the average of "V"C1 is "V"in. Since C1 blocks DC current, the average of "I"C1 is zero. Since average "I"C1 is zero, the only source of the average load current is "I"L2. Therefore, the average current through L2 is the same as the average load current and is independent of the input voltage.

Looking at average voltages, the following can be written: V_{IN} = V_{L1} + V_{C1} + V_{L2}

and since 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 decreases (becomes more negative). 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 decrease (more negative) the current "I"L2.

When switch S1 is turned off, the current IL1 becomes the same as the current "I"C1. Also, since inductors do not allow instantaneous changes in current, the current "I"L2 will continue in the negative direction. From 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 L2 and L1. C1, however is being charged by L1 during this off cycle, and will in turn recharge L2 during the on cycle.

The capacitor CIN 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

Non-ideal circuit

The voltage drop and switching time of the diode (D1) is extremely critical. The 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 silicon or Schottky diodes can be used. The resistances in the inductors and the capacitors can also have large effects on the converter efficiency and ripple. Lower series resistance in the inductors allows for less energy dissipated as heat, which results in a large portion of the energy being transferred to the load. Low ESR capacitors 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.

ee also

*Boost converter
*Buck converter
*Buck-boost converter
*DC to DC converter
*Switched-mode power supply (SMPS)

External links

Excellent Design Guidelines from National Semiconductor in [ Application Note 1484]


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

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

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • SEPIC — Ein SEPIC Wandler (Abkürzung für englisch single ended primary inductance converter) ist in der Elektronik eine Form von Gleichspannungswandler. Die positive Eingangsspannung UE des SEPIC Wandlers kann sowohl größer als auch kleiner als die… …   Deutsch Wikipedia

  • Ćuk converter — Cuk redirects here. For the Idel Ural festival, see Çük. The Ćuk converter (pronounced Chook, sometimes incorrectly spelled Cuk, Čuk or Cúk) is a type of DC DC converter that has an output voltage magnitude that is either greater than or less… …   Wikipedia

  • Convertisseur SEPIC — Figure 1: Schéma de base d un convertisseur SEPIC Un convertisseur SEPIC de l acronyme de (single ended primary inductor converter) est une alimentation à découpage convertissant une tension continue en une autre tension continue, de valeur… …   Wikipédia en Français

  • DC-to-DC converter — An assortment of 78xx series ICs, linear DC DC converters A DC to DC converter is an electronic circuit which converts a source of direct current (DC) from one voltage level to another. It is a class of power converter. Contents …   Wikipedia

  • Gleichspannungswandler — Ein Gleichspannungswandler, auch DC DC Wandler genannt, englisch DC DC Converter, bezeichnet eine elektrische Schaltung, welche eine am Eingang zugeführte Gleichspannung in eine Gleichspannung mit höherem, niedrigerem oder invertiertem… …   Deutsch Wikipedia

  • Switched-mode power supply — switched mode power supply. A bridge rectifier B Input filter capacitors C Transformer D output filter coil E output filter capacitors ] A switched mode power supply, switching mode power supply or SMPS, is an electronic power supply unit (PSU)… …   Wikipedia

  • Schaltregler — als Begriff der Elektrotechnik bzw. Elektronik bezeichnet Techniken zur Spannungs und/oder Stromwandlung als Basis für die Stromversorgung elektrischer Geräte und Baugruppen mithilfe eines periodisch arbeitenden elektronischen Schalters und… …   Deutsch Wikipedia

  • Convertisseur Flyback — Fig. 1 : schéma de base d un convertisseur Flyback. Exempl …   Wikipédia en Français

  • Zeta-Wandler — Ein Zeta Wandler ist in der Elektronik eine Form von Gleichspannungswandler, der eine zugeführte positive Gleichspannung entweder in eine betragsmäßig höhere oder niedrigere positive Ausgangsgleichspannung umwandeln kann. Anwendungen dieses… …   Deutsch Wikipedia

  • Ćuk-Wandler — Als Ćuk Wandler (englisch Ćuk converter) bezeichnet man in der Elektronik eine diskrete elektronische Schaltung, die eine elektrische Gleichspannung in eine andere elektrische Gleichspannung transformieren kann. Der Betrag der… …   Deutsch Wikipedia