Understanding TL431 Operation - Basic Operation and Power Supply Compensation

Understanding TL431
Operation
Basic Operation and Power Supply Compensation
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Purpose
The purposes of this presentation are:
1. Understanding how the TL431 works as an adjustable zener diode
2. Understanding how the TL431 works as a compensator in the feedback loop
of the switching converters

Assumptions
● You have to be aware of power supply feedback loop analysis
● This presentations does not discuss how to compensate the power supply,
but only explains how the compensator is implemented using TL431

TL431 Basic Operation
This is the circuit
symbol. It looks
like an adjustable
zener diode
This is the
internal structure

This is the circuit
symbol. It looks
like an adjustable
zener diode
This is the
internal structure
How does this circuit
behave as an adjustable
zener diode ??? linkedin.com/in/mohammedfouly

vref
vout
Rload
vout = vref
Iload
If you want to supply a load with a certain voltage value, simply obtain a voltage
source with the desired value and directly connect it to the load.
The load current is totally consumed from the voltage source Vref in our case.

--
+
vref
vout
Rload
vout = vref
Iload
?
If you want to keep the
reference voltage unloaded,
simply use an Op-Amp in the
buffer topology.
Keep in mind that the Op-Amp
should have the current driving
capability needed to drive the
load.
From where can the Op-Amp
supply this load current?

--
+
vref
vout
Rload
vout = vref
Iload
From the supply rails.
You need to supply the
buffer with another voltage
source VDC that is capable of
providing the required load
current.
It is not required from VDC to
be a regulated source, as
the regulation task is
performed by the buffer with
the aid of the voltage
reference VrefvDC

--
+
vref
vout
Rload
vout = vref
Iload
The Op-Amp output Vout is
only dependent on input
voltage Vref
Vout is almost independent
on the supply VDC
i.e. the Op-Amp rejects any
variations of its power supply
VDC from affecting its output
Vout
i.e. the Op-Amp has a large
Power Supply Rejection
Ratio (PSRR)
vDC

--
+
vref vDC
vout
Rload
Rup
Rdn
vout = vref (1+Rup/Rdn)
Iload
Simple modification to adjust
Vout to any desired value
different to Vref by just
selecting the proper values
for Rup and Rdn

--
+
vref vDC
vout
Rload
Rup
Rdn
Iload
Simple modification to adjust
Vout to any desired value
different to Vref
To continue the explanation, it is needed to
say a short word about Op-Amp anatomy

TL431 Basic Operation - Op-Amp anatomy
--
+
The Op-Amp consists of two stages:
1- Differential amplifier stage
2- Output stage
Differential
Amplifier
Output
Amplifier
--
+
vout
vout
vin_n
vin_n
vin_p
vin_p
Main functions of
differential amplifier stage:
1- Large input impedance
2- Large differential gain
Main functions of output stage:
1- Large output voltage swing
2- Large output current capability
vmid

--
+
vout
vout
Implementation of
differential amplifier
stage
Implementation
of output stage
vin_n
vin_p
vin_n
vin_p
vmid
2- Output stage

--
+
vout
vout
Gain of differential
amplifier stage = Ad
Gain of output
amplifier stage = -Ao
vin_n
vin_p
vin_n
vin_p
vmid
2- Output stage

--
+
vout =-Aovmid
vout
Gain of output
vmid=Ad[(vin_d+)-(vin_d-)]
vin_n
vin_p
vin_n
vin_p
2- Output stage

--
+
vout =-Aovmid
vout
-Ao = Gain of output
amplifier stage
vin_n
vin_p
vin_n
vin_p
Note:
The node Vmid sees the node Vin_d+ as non-inverting input
The node Vmid sees the node Vin_d- as inverting input
2- Output stage

--
+
vout =-Aovmid
vout
Gain of output
vout = -AoAd[(vin_d+) - (vin_d-)]
vin_n
vin_p
vin_n
vin_p
Overall
Gain
2- Output stage

--
+
vout =-Aovmid
vout
Gain of output
vin_n
vin_p
vin_n
vin_p
vout = +AoAd[(vin_p) - (vin_n)]
Overall
Gain
2- Output stage

--
+
Adding the reference voltage source
vout
vout
vmid
vref
vin_n
vin_p
vin_n
vin_p

--
+
vout
vout
vmid
vref
All components inside the dashed line
can be encapsulated into a single
chip of 4 terminals
1- Ground
2- Open drain output
3- Inverting input with respect to Vout
4- Supply
vin_n
vin_p
vin_n

--
+
All components inside the dashed line
can be encapsulated into a single
chip of 3 terminals
1- Ground
4- Supply
vout
vout
vmid
vref
Keeping in mind that the
differential amplifier has
large PSRR, we can supply
it from the Vout without
degradation in performance
and the terminals count will
reduce to only 3
vin_n
vin_p
vin_n

1- Ground
vout
vout
vmid
vref
Redrawing the symbol
Non-inverting input with
respect to Vmid
--
+vref
--
+
GND
vmid
vin_n
vin_n

--
+
1- Ground
vout
vout
vmid
vref
vref
--
+
GND
vmid
respect to Vmid
Original
symbolic
structure
vin_n
vin_n

--
+
1- Ground
vout
vref
--
+
GND
vmid
respect to Vmid
Original
symbolic
structure
● Diode added for protection against reverse connection
● BJT technology used in manufacturing
● Terminals renaming
vin_n

--
+
vref vDC
vout
Rload
Rup
Rdn
Iload
This is the desired circuit.
Implemented using Op-Amp

voutvmid
vref
vin_n vDC
Rload
Rup
Rdn
--
+
vref vDC
vout
Rload
Rup
Rdn
Iload
This is the desired circuit.
Implemented using TL431

--
+
vout
vout
vmid
vref
vref
--
+
GND
vmid
vin_n vDC
Rload
Rup
Rdn
--
+
vref vDC
vout
Rload
Rup
Rdn
Iload
Rup
Rdn
vin_n
vDC
Rload
RD
Same circuit, another
symbol for TL431

--
+
vref vDC
vout
Rload
Rup
Rdn
Iload
Rup
Rdn
vout
Rload
Iload
vDC
RD
Load current is steered
totally out of the chip
Same circuit, another
symbol for TL431

TL431 Basic Operation vout = vref (1+Rup/Rdn)
Rup
Rdn
vout
Rload
Iload
vDC
RD
Rup
Rdn
vout
Rload
Iload
vDC
RD
Acts as adjustable zener diode

--
+
vref vDC
vout
Rload
Rup
Rdn
Iload
Rup
Rdn
vout
Rload
Iload
vDC
RD
Can you justify why is it not
positive feedback?
It looks as a positive feedback !!

TL431 in Feedback Loop
In some power supply topologies it is required to close the feedback loop using an
electrically isolated path.
TL431 is perfect in such function
Now, a circuit is desired to perform the following functions:
1. Generating an output voltage that represents the disturbance in the input
voltage via a specific transfer function
2. The generated output voltage is electrically isolated from the input voltage

In some power supply topologies it is required to close the feedback loop using an
electrically isolated path.
TL431 is perfect in such function
Now, a circuit is desired to perform the following functions:
1. Generating an output voltage that represents the disturbance in the input
voltage via a specific transfer function
2. The generated output voltage is electrically isolated from the input voltage
This transfer function presents the desired
poles and zeros for compensation process.
The compensation techniques are out of
scope of this presentation.

These functions will be achieved by 2 steps:
1. Using opto-isolator circuit
2. Modifying the TL431 circuit explained previously

TL431 in Feedback Loop - Opto-isolator basic operation
It will be required to make some AC analysis based on the small signal model of
the Opto-isolator

RdRc
vDC1vDC2
vAC2
viso vAC1
VDC1 and VDC2 are for biasing
VAC1 and VAC2 represents the
voltage disturbances

RdRc
vAC2
viso vAC1
rd
CTR
id
ic
Small signal model of the
opto-isolator
The diode is modeled by a
very small resistor which can
be neglected compared to Rd
The transistor is modeled by a
dependent current source
equals the diode current times
the CTR (Current Transfer
Ratio) of the Opto-isolator

RdRc
vAC2
viso vAC1
CTR
id
ic
viso = CTR(Rc/Rd)[vAC2 - vAC1]
With simple circuit analysis
we can derive that:

Now, before combining the TL431 in the feedback loop we need to modify its
circuit to operate as a complete Op-Amp

-
Inverting
Input
Output
GND
Supply
Rbias
TL431
Imagined
Op-Amp
Keep in mind that
the non-inverting
input is internally
connected to the
reference voltage
Non-inverting
Input

-
Output
GND
Supply
Rbias
Rup
Rdn
Zfb
vout = -vin (Zfb/Rup) vin
vout
Vref is zero during the AC
analysis, so the inverting
input of the Op-Amp is
virtually ground

Rup
Rdn
Zfb
vout = -vin (Zfb/Rup)
vin
vout
Redrawing the circuit without the
imagined Op-Amp Symbol
Rbias
vsupply

Rup
Rdn
Zfb
vout = -vin (Zfb/Rup)
vout
This circuit is aimed mainly to
monitor the supply voltage. So, Vin
is replaced by Vsupply
Rbias
vsupply

Rup
Rdn
Zfb
vout = -vsupply (Zfb/Rup)
vout
These 2 circuits should be
combined into single circuit
Rbias
vsupply
RdRc
vAC2
viso
vAC1
CTR
id
ic
viso = CTR(Rc/Rd)[vAC2 - vAC1]

Rup
Rdn
Zfb
vout
Vout will be in place of VAC2
Vsupply will be in place of VAC1
Rbias will be in place of Rd
Rbias
vsupply
RdRc
vAC2
viso
vAC1
CTR
id
ic
viso = CTR(Rc/Rd)[vAC2 - vAC1] vout = -vsupply (Zfb/Rup)

viso
CTR
id
ic
Rup
Rdn
Rbias
vsupply
Rc
Making the above substitutions, the
output voltage will be a function of the
supply voltage as the equation below
Vout will be in place of VAC2
Vsupply will be in place of VAC1
Rbias will be in place of Rd
Zfb

viso
CTR
id
ic
Rup
Rdn
Rbias
vsupply
vDC2
Rc
Redrawing the circuit with the
original symbols and biasing
Zfb

viso
CTR
id
ic
Rup
Rdn
Zfb
Rbias
vsupply
vDC2
Rc
This is the basic circuit and its
related transfer function that can
be tuned to match the
compensation requirements

Compensators are designed to be one of 3 types:
Type I : Transfer function contains a single pole at the origin

Type II : Transfer function contains a single pole at the origin, a single pole at
higher frequency and a zero

Type III : Transfer function contains a single pole at the origin, a pair of poles at
higher frequency and a pair of zeros

viso
CTR
Rup
Rdn
Rbias
vsupply
vDC2
Rc
This circuit and its related
transfer function that can be
tuned to match the compensator
types as follows
Zfb

viso
CTR
Rup
Rdn
Rbias
vsupply
vDC2
Rc
To obtain type I compensator,
Zfb should be a capacitor
The transfer function will be very
similar to the desired one
The effect of this
extra zero can be
suppressed easily
C

viso
CTR
Rup
Rdn
Rbias
vsupply
vDC2
Rc
To obtain type II compensator, extra
capacitor should be connected
across the transistor terminals
The transfer function will be similar
to the desired one
Cc
Using small signal model of the circuit
leads to finding Rc is parallel to Cc
C

viso
CTR
Rup
Rdn
Rbias
vsupply
vDC2
Rc
To obtain type III compensator, extra
R-C branch should be connected
parallel to Rbias
The transfer function will be similar
to the desired one
Cc
C2
R2
C

References
● ON Semiconductor “The TL431 in the Control of Switching Power Supplies”
● ResearchGate “Designing with the TL431 - the first complete analysis”

Understanding TL431 Operation - Basic Operation and Power Supply Compensation

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