How to configure a Point of Load converter (POL) for negative or isolated output voltages

In harsh environment applications where temperatures are as high as 210 degrees C or when a radiation tolerant solution is required, an integrated buck solution would best meet the system needs. There are numerous applications where a negative output voltage, or an isolated output voltage such as +12V or +15V, is required to provide power for MOSFET gate drives or to bias an op-amp.  In this blog, we will explore how to configure a buck converter using the TPS50x01 to provide negative output voltage. We will also cover how to provide voltage that is higher than input voltage to meet application needs.

The TPS50601-SP and TPS50301-HT are integrated synchronous buck converter solutions targeted for harsh environments, such as radiation tolerant, geological, heavy industrial, and oil and gas applications.  The TPS50x01 is a current mode control with integrated high and low side MOSFETS.  A large thermal PAD part of the IC keeps thermal management in check to distribute heat evenly on the PCB.  Pvin range is from 1.6V to 6.3V, whereas Vin voltage powering the control circuitry has range from 3V to 6.3V.

 synchronous buck converter
Figure 1: synchronous buck converter

As shown in Figure 1, the synchronous buck converter output voltage is stepped down:

Vout = Vin*D where D is duty cycle.

I recommend using the TPS50x01 for applications that need an output voltage higher or lower than the input voltage, but with opposite polarity, as in the case of buck-boost converter. The  TPS50x01 can be configured as a buck-boost converter by referencing the IC to Vout (-Ve output Voltage) as shown below.

 TPS50x01 configured as buck-boost converter
Figure 2:  TPS50x01 configured as buck-boost converter

Since the rating of MOSFETS in the TPS50x01 is limited to 6.3V, the Vin max + Vout must be less than 6.3V.  Vin min must be greater than 3V if a single input voltage is used that is Pvin = Vin.

In applications where we have output voltage greater than input voltage, the TPS50x01 can be configured as a Fly-Buck converter.  This is a simple buck-like design with low part counts as shown below in Figure 3. 

 TPS50X01 configured as Fly-buck converter
Figure 3: TPS50X01 configured as Fly-Buck Converter

The output inductor is replaced with a coupled inductor T1, and voltage across the Cr is regulated by the control loop.   Output voltage is a reflection of voltage across Vr. It is modified by coupled inductor turns ratio, minus a diode drop.  Secondary winding voltage is rectified by diode D1 and capacitor Co.

Output voltage can be expressed in the below equation, where D is duty cycle of converter.

Vout = Vin x D(Nsec/Np) – Vd1           

An additional auxiliary winding is added to the coupled inductor T1 when multiple outputs are required.  Since the secondary windings are isolated, outputs are configured as positive or negative.  Grounding the positive output node results in negative output polarity.

No current flows in the secondary winding of T1 when the main switch S1 is on.  Primary and secondary currents exhibit resonant behavior.  This is due to resonant frequency formed by Cr, Co and the leakage of the coupled inductor T1.

In Fly-Buck topology, load regulation is degraded by leakage inductance of the coupled inductor, diode drop and DCR of the coupled inductor winding.  Minimizing leakage inductance and the DCR of the coupled inductor provides improved load regulation.

Check out this applications report on creating an inverting power supply using a synchronous step-down regulator for more information.