Power Supply Specifications

As the members of the group are from different final year project groups with disparate requirement of power supply requirements, we choose to focus on a general application.  The output voltage is expected to be 5-12v (11.5v for demonstration purposes), 1A current at 1% output voltage fluctuation.

Conspectus

The Buck-boost converter has the following configuration.

Figure 1 Basic Buck Converter

To provide a robust controlling that is capable of producing varied output voltages in the range of 5 – 12v independent of input voltages, TPS40200 voltage mode controller IC has been chosen.  Other factors that influenced the choice are following.

1.      Availability of simulation tool (Tina) and libraries for the controller IC

2.      Ability to procure the components (Texas instruments’ components)

3.      Robust, versatile IC

4.      Ability to support wide variety of outputs independent of input

5.      Higher current handling capability

6.      Higher input / output voltage handling capability

 

The TPS420

Figure 2 TPS40200 Pin Layout

TERMINAL				I/O	DESCRIPTION
RC			1	In	Switching frequency setting RC network.  It may be synchronized to an external clock by connecting an open drain output to this pin and pulling it to GND
SS			2	In	Soft-start programming pin. Pulling this pin below 150 mV causes the output switching to stop, placing the device in a shutdown state. The pin also functions as a restart timer for over current events.
COMP			3	Out	Error amplifier output.
FB			4	In	Error amplifier inverting input.
GND			5	-	Device ground.
GDRV			6	Out	Driver output for external P-channel MOSFET
ISNS			7	In	Current-sense comparator input.
VDD			8	In	System input voltage.

TPS40200 is a flexible voltage mode controller with a built in 200-mA driver for p-channel FETS. It has input voltage range up to 52v and soft-start, current limiter and variable clock frequency options. Also being a Texas Instrument component it has the wide availability and boasts abundant support materials.

A block diagram of the internal configuration of the IC is given below

Figure 3-IC internal Configuration

The operation of the controller and the rest of the circuit is analyzed along with the calculations of the component values with appropriate references to the datasheet of TPS40200.

Figure 3 Basic Schematic with IC

For demonstrational purposes, 11.5v output is selected and to generate around 1A output 10 Ω load resistor is added which makes the real average output current 1.15 A (in 1-2A range). Further to accommodate smaller inductors 450 kHz operating frequency is chosen, as the controller is capable generating programmed output frequency from 30 to 500 kHz (Annexure A1).

Clock to the controller

Figure 4 Oscillator Functional Digram

Equation 1 Operating Frequency

  Tuning R_RC and C_RC will produce the desired output frequency. However the following requirement must be satisfied according to the datasheet.

 

Equation 2 Clock resistor

47k is chosen for R_RC which yields

V_in / R_RC                     = 510 µA < 750 µA

which results in

C_RC                   = 1 / [ f x R_RC  x 0.105 ]

                         = 1 / [ 450000 x 47000  x 0.105 ]

                        =  450 nF

Both values give real operating frequency

f                       = 1 / [ 0.105 x R_RC  x C_RC ]

                        = 1 / [0.105 x 47000 x 450 x 10^-9 ]

                        = 450.3 kHz

• Output voltage (V_out) : 11.5 V
• Load resistor (R_L) : 10 Ω
• Output current (I_out) : 1.15 A
• Operating Frequency (f) : 450.3 kHz

Selection of Inductor

Inductor peak current variation (∆I_L)

 ∆I_L is taken to be 15% of the output current which is 1.15A.

             ∆I_L        = 0.15 x 1.15

                        = 0.1725A

∆I_L       = (V_in x V_out) / [ ( V_out  + V_in) x f x L ]

L          = (V_in x V_out) / [ ( V_out  + V_in) x f x ∆I_L ]

            = (12 x 24) / [ ( 12  34) x 450300 x 0.1725 ]

            = 102.99 µH

100 µH was chosen for L (an acceptable value when the maximum current ratings of the inductors are considered)

Results in actual ∆I_L

∆I_L              = (V_in x V_out) / [ ( V_out  + V_in) x f x L ]

= (12 x 24) / [ ( 12  34) x 450300 x 100 x 10^-6 ]

= 0.178 A

Selection of Capacitors

 

TPS40200 employs a soft-start which is essential as it has inbuilt short circuit protection which will shut down the circuit when higher current is detected and upon which a restart is needed.

 

Figure 5 Soft-start circuit

 

 

Equation 3 Output LC oscillation

 

According to the datasheet the soft-start time should be slower than LC oscillation and it is given by

Equation 4 Soft-start time

 

1.      Hence 10 µF is chosen for C_o which gives t_s minimum

            Ts        > 2 π x ( L x C_o)^0.5

                                                > 2 π x ( 100 x 10^-6 x 10 x 10^-6)^0.5

                        >  199 µs

 

Which  results in Css minimum of

C_ss        > T_{s, min /}  R_c x ln (8 / 8 – 1.4)

>  105000 x ln R_c x ln (8 / 8 – 1.4)

> 9.85 nF

 

2.      To be safer 18 nF is chosen for C_ss

Implementation of short-circuit protection

 

TPS40200 comes with a built in current limiter.  As shown in Figure 3, a resistor in series with the power MOSFET sets the over current protection level. When the FET is on and the controller senses 100mV or more drop from the VDD pin to the ISNS pin, an over current condition is declared. When this happens, the FET is turned off, , the soft-start capacitor is discharged. When the soft-start capacitor reaches a level below 150 mV, the converter clears the over current condition flag and attempts to restart.

 

 

Figure 6 Current Limiter

 

 

Equation 5 Selection of Rlim

 

 

To allow upto 2A in I_oc, R_lim is chosen at unit scaling

R_lim                  =  0.1 / 2

                        = 0.05 mΩ

 

Voltage Gain

 

To support multiple output voltages independent of input voltage TPS40200 employs an internal voltage comparator and an offset (V_ss) as shown in figure 2. Further this offset is checked between the V_out and V_ref to set the PWM duty cycle.

 

Figure 7 Error amplifier

 

 

Equation 6 Modulator gain

 

Hence for a V_out of 11.5

            R₂ / R₁                                    =  11.5 / 0.708 – 1

                                    = 15.24

 

Having chosen 24 K for R₂

            R₁                    = 24 K / 15.24

                                    = 1.57 K

 

Having chosen a 1.54 K resistor for R₁,

            V_out                  = 0.708 x [1 + 24 / 1.54]

                                    = 11.74 v

                         

 

Selection of FET

 

Considering the fact that the operating frequency is high as 450 KHz, and TPS40200 produces the gate output for P-Channel, we chose FDS4685 40V P-Channel Power Trench MOSFET owing to its properties given below in Table 4.

 

Table 2 Electrical characteristics of FDS4685

 

Selection of rectifier diode

 

High frequency, high power diode is needed and more specifically

Pulse width                = 1/ f

                                    = 1/ 450300

                                    = 2.2 µs

Hence we need a recovery time that is shorter than at least 0.1 µ, i.e. about 1/20^th of the clock cycle allowing 20% duty cycle condition which is possible for voltage as low as 5v.

 

The maximum reverse voltage applied across the diode

Input voltage             = 24V

 

D                                 = V_out / [ V_out + V_in]

                                    = 11.5 / [24 + 11.5]

                                    = 0.324

 

The diode average current,

I_D avg                          = I_out (T- t)/ T

                                    = I_out (1 - D)

= 1.15 (1- 0.324)

= 0.778A

 

 

This compelled us to go for high current density schottky barrier rectifiers.  The characteristics given in Table2 justify our selection of SS10P4.

 

Table 3 Characteristics of SS10P4

 

Table 4 Electrical characteristics

 

Further Table 3 shows us that it has a very low junction capacitance at 1MHz which enables the fast recovery (Annexure B1).

 

Miscellaneous components and the list of parameters

 

By consulting the datasheet and sample applications mainly given for buck type power supply, other resistor, capacitor values were chosen and they played only a trivial role in the simulation

List of parameters

• Output voltage (V_out) : 11.5 V
• Load resistor (R_L) : 10 Ω
• Output current (I_out) : 1.15 A
• Operating Frequency (f) : 450.3 kHz
• Output current variation (∆I_L) : 0.178 A
• L : 100 µF
• Css : 15 nF
• R_lim : 0.05 mΩ
• R₁ : 1.57 K
• R₂ : 24 K
• Output voltage (V_out) : 11.74 V

Simulation

 

Being fortunate enough, Texas Instrument’s custom simulation software Tina contained the library modules for TPS40200.

 

Figure 8 Screen shot of the application

 

·        Tina^TM  was obtained from Texas Instruments
http://dl-www.ti.com/lit/sw/sloc058d/sloc058d.zip

 

·        Tina is a light weight pspice tool for Texas Instrument with built in libraries for most of the Texas Instruments, vishey and Fairchild semiconductor components.

·        It is capable of generating simulated output in Time and Frequency domain, DC analysis report, ac analysis report and bill of materials.

 

Circuit diagram

 

·         Netlist file generated by Tina is given in Annexure C1.

·         Bill of Materials is given in Annexure C2.

·        

 

Simulator Results

The simulation results have been confirming our calculations.

·         The following simulation graphs depict the state between 690 µs and 700 µs

 

 

RC output signal

Clock

Gate Voltage

 

Error comparator voltage

 

Inductor Current

 

Output Voltage

 

Output Current

 

All in one

 

Transient state

 

The following simulation graph depicts the state between 0s and 10ms, which demonstrates the transient output. Further it is clear that it takes around 400 µs to attain the steady state which was calculated above.

 

 

References

 

• 1.      Datasheet of FDS4685  40V P-Channel PowerTrench MOSFET

• 2.      Datasheet of SS10P3 & SS10P4 Vishay General Semiconductor

• 3.      Datasheet of TPS40200 Texas instrument voltage mode controller

• 4.      Positive to Negative Buck-Boost Converter Using LM267X SIMPLE SWITCHER® Regulators – National Instruments

• 5.      User's Guide Using the TPS40200 174A – August 2006 – Revised August  - Texas Instruments

• 6.      Course Notes

• 7.     http://www.TI.com