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A Unique Combination UP/DOWN SMPS

When designing at the board level it sometimes becomes necessary to generate a constant output voltage that is less than that of the battery. The step–down circuit shown in Figure 1a will perform this function efficiently. However, as the battery discharges, its terminal voltage will eventually fall below the desired output, and in order to utilize the remaining battery energy the step–up circuit shown in Figure 1b will be required.

Figure 1

By combining circuits a and b, a unique step–up/down configuration can be created (Figure 2) which still employs a simple inductor for the voltage transformation. Energy is stored in the inductor during the time that transistors Q1 and Q2 are in the "on" state. Upon turn–off, the energy is transferred to the output filter capacitor and load forward biasing diodes D1 and D2. Note that during ton this circuit is identical to the basic step–up, but during toff the output voltage is derived only from the inductor and is with respect to ground instead of Vin . This allows the output voltage to be set to any value, thus it may be less than, equal to, or greater than that of the input. Current limit protection cannot be employed in the basic step–up circuit. If the output is severely overloaded or shorted, L or D2 may be destroyed since they form a direct path from Vin to Vout . The step–up/down configuration allows the control circuit to implement current limiting because Q1 is now in series with Vout , as is in the step–down circuit.

Figure 2

You can build this device by using main component MC34063 Universal Switching Regulator Subsystem from ON-Semiconductor. A complete step–up/down switching regulator design is shown in schematic. An external switch transistor was used to perform the function of Q2. This regulator was designed to operate from a standard 12 V battery pack with the following conditions:

Vin = 7.5 to 14.5 V

Vout = 10 V

fmin = 50 kHz

Iout = 120 mA

Vripple(p–p) = 1% Vout or 100 mVp–p

The following design procedure is provided so that the user can select proper component values for his specific converter application. Determine the ratio of switch conduction ton versus diode conduction toff time:

The cycle time of the LC network is equal to ton(max)+ toff :

Next calculate ton and toff from the ratio of ton /toff in #1 and the sum of ton(max) + toff in #2:

The maximum on–time is set by selecting a value for Ct:

The peak switch current is:

A minimum value of inductance can now be calculated since the maximum on–time and peak switch current are known.

A 120 mH inductor was selected for Lmin. You can build your own inductor, see Designing Inductor for DC DC Converter

A value for the current limit resistor, Rsc, can be determined by using the current limit level of Ipk(switch) when Vin = 14.5 V.

A minimum value for an ideal output filter capacitor is

Ideally this would satisfy the design goal, however, even a solid tantalum capacitor of this value will have a typical ESR (equivalent series resistance) of 0.3 W which will contribute an additional 209 mV of ripple. Also there is a ripple component due to the gain of the comparator equal to:

The ripple components are not in phase, but can be assumed to be for a conservative design. From the above it becomes apparent that ESR is the dominant factor in the selection of an output filter capacitor. A 330 mF with an ESR of 0.12 W was selected to satisfy this design example by the following:

The nominal output voltage is programmed by the R1, R2 resistor divider.

If 1.3 k is chosen for R1, then R2 would be 9.1 k, both being standard resistor values.

Transistor Q1 is driven into saturation with a forced gain of approximately 20 at an input voltage of 7.5 V. The required base drive is:

The value for the base–emitter turn–off resistor RBE is determined by:

A standard 300 W resistor was selected. The additional base current required due to RBE is:

The base drive resistor for Q1 is equal to:

A standard 150 W resistor was used.

 

Schematic :

 

 

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Any suggestions, comments, etc. E-mail me: fridiant@yahoo.com

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