Type 1, Unidirectional Current Transformers
This first type measures unidirectional current pulses, such as output rectifier diode cur-
rents, switching regulator currents, and the currents that would flow in the primary or
secondary of a forward converter transformer.
Type 2, AC Current Transformers
In the second type, the transformer is used to indicate ac currents where the measured cur-
rent flows equally in both directions and there is no net DC component. A typical applica-
tion would be to measure the current in series with the primary winding of a half-bridge
Type 3, Flyback-Type Current Transformers
This type of current transformer is used in the flyback mode, and is particularly useful for
applications in which the current pulse can be very narrow.
Type 4, DC Current Transformers
This very useful, less well known DC current transformer can be used to measure current
in high-current DC output lines, with very low loss.
14.3 CORE SIZE AND MAGNETIZING
CURRENT (ALL TYPES)
Selecting the core size is probably the most difficult part of the design exercise. A compro-
mise must be struck between the ideal performance and practical considerations of size,
cost, and number of turns.
In general, for current transformers, the larger the inductance, the smaller the mag-
netizing current and the more accurate the measurement. The magnetizing current com-
ponent increases during the pulse duration and will be subtracted from the quantity to
be measured. Consequently, at the end of the conduction pulse, the magnetizing current
should be small compared with the measured quantity. For current limit applications, a
magnetizing current of 10% is a typical design limit. This magnetization effect is most
easily shown in the unidirectional current transformer.
Figure 3.14.1a shows a typical unidirectional current transformer and secondary cir-
cuit. This would be used for current limiting in, say, a forward converter. The primary has
a single turn on a toroidal core. (This primary turn is often a connecting lead or output bus
that is simply passed through the center of the toroid.) The primary turn is thus in series
with the line to be monitored.
The secondary has a large number of turns, which are terminated in a ballast resistor R2
via a diode D1. The intention is that a true voltage analogue of the primary forward current
pulse be developed across R2. (D1 blocks the reverse recovery voltage.) However, it will
be seen from Fig. 3.14.1b and c that the secondary waveform is distorted as a result of the
magnetizing current component.
Figure 3.14.1b shows the applied unidirectional (all positive) primary current pulse.
Figure 3.14.1c shows the corresponding secondary voltage pulse developed across R2.
14. CURRENT TRANSFORMERS
The effect of two values of secondary magnetizing current, a small value I
and a large
, shows how the magnetizing current is effectively subtracted from the ideal
transformed current analogue I
. It will be clear from this diagram that if the peak value
of the current at the end of the conduction pulse, `I
, is to be useful for current-limiting
purposes, then the secondary inductance of the current transformer must be high enough to
ensure that at least a positive slope remains on the net secondary waveform. This means that
a large secondary inductance is needed, and so a large number of secondary turns, a large
core, and high-permeability core material are required. In general, the largest-permeability
core material will be used, leaving a trade-off between turns and core size.
Note: Losses in amplitude resulting from winding resistance, diode loss, and magnetizing
current amplitude can be corrected to some extent by adjusting R2, provided that the slope
A second major factor that influences the current transformer magnetizing current will
be the magnitude of the secondary voltage. This voltage is the sum of the selected signal
(say 200 mV in this example) and the D1 rectifier diode forward voltage drop
(say 0.6 V). The secondary voltage should be as small as possible (consistent with a good
signal-to-noise ratio), since large values of V
will result in large magnetizing currents. To
this end, small schottky diodes should be considered for D1.
If a very small toroid is chosen for the current transformer, then to get the required
inductance, a large number of secondary turns will be required. If the number of second-
ary turns is too large (say in excess of 200), then there will be significant interwinding
capacitance, and the high-frequency response (response to narrow current pulses) will be
Hence, the core size is a compromise between cost and performance. A good compro-
mise is to select a core that will require approximately 100 turns on the secondary (in a
single layer) and will give the required minimum inductance.
FIG. 3.14.1 (a) Current transformer and secondary circuit used for unidirectional current pulse
measurement. (b) Applied unidirectional primary current waveform, and (c) developed secondary
current waveforms on R2, showing the effect of current transformer magnetization current.
14.4 CURRENT TRANSFORMER DESIGN
A unidirectional current transformer would be used for monitoring the discontinuous cur-
rent pulses in, say, the output rectifiers of a high-current forward converter or the primary
current of a single-ended forward converter. A typical application is shown in Fig. 3.14.2a,
where the current transformer T1 is in series with the primary of a single-ended forward
converter. The design steps are as follows:
Calculate (or observe), the peak primary current to be measured and the slope di/dt on the
top of the current waveform. This will be used to calculate the minimum current trans-
FIG. 3.14.2(a) Basic circuit of a single-ended forward converter, showing a uni-
directional current transformer in the primary of the main power transformer.
(b) Primary current and secondary voltage waveforms of current transformer,
together with the current analogue signal voltage developed across R2. This may
be used for current-mode control and current limiting.
14. CURRENT TRANSFORMERS
Select the current transformer secondary voltage at the limiting current value. (This should
be kept as low as possible, typically <1 V including the diode drop.)
Select a high-permeability core material and initial size.
Note:For unidirectional current pulse applications, the core material should have two prop-
1. High permeability, so that a large inductance will be obtained for the minimum number
2. A low remanence B
, so that the core will restore to a low flux level when the current
pulse drops to zero. This will ensure that the core will not saturate after a few cycles of
Unfortunately, these two requirements tend to be mutually exclusive, so a compromise
choice must be made. A material such as H5B2 (Fig. 2.15.4b) would be a good compro-
mise. The core should be insulated to reduce capacitance between the winding and the core
and thereby reduce interwinding capacitance.
At larger currents, the physical requirements for the prmary wire diameter may define
the minimum core size. If a large output voltage is required, then it is recommended that
a larger core be used, allowing a greater number of secondary turns. For practical reasons,
the primary will be maintained at one turn (that is, the primary wire is passed straight through
the center hole of the toroid).
14.5 UNIDIRECTIONAL CURRENT
TRANSFORMER DESIGN EXAMPLE
Figure 3.14.2b shows the waveforms that would be expected from a current moni-
toring transformer of type 1 (unidirectional pulse type) in the forward converter of
In this example, when the primary power transistor is “on,” the forward current in T1
takes the starts of all windings positive, and secondary diode D1 conducts. The current
in R2 will be a transform of the primary current, and an analogue voltage of the primary
current will be developed across R2.
When Q1 turns off at the end of the forward current pulse, rapid recovery of the cur-
rent transformer core occurs because D1 blocks and the secondary flyback load resistance
R1 is high. As a result, the flyback voltage is large, and this gives a rapid core restoration
between forward pulses. That is, the flux density returns to the residual value B
“off” period, ready for the next forward pulse.
Note:This rapid core restoration allows closely spaced forward current pulses to be moni-
tored accurately, without saturation of the core. Clearly, the value of R1 will be chosen to
get the required minimum recovery time, and the voltage rating of D1 must be selected to
block the reverse flyback voltage across R1.
Step 1, Calculate the Primary Ampere-Turns
In this example, the primary current is 10 A in a single turn. This gives a primary magnetiz-
ing force of 10 ampere-turns due to the forward forcing current that flows in the primary of
the forward converter transformer shown in Fig. 3.14.2.
Step 2, Define Secondary Turns and Calculate Secondary Current
This design will use the preferred winding not exceeding 100 turns of #34 AWG in a single
layer (to give a small core size and good high-frequency performance). Therefore, to pro-
vide an equal depolarizing magnetic force of 10 ampere-turns, the secondary current will
be 100 mA. Since #34 has a current density of 450 A/cm2 at 91 mA, this gauge is a good
choice for 100 mA.
Step 3, Define the Required Secondary Voltage
The secondary voltage will be 0.8 V, made up of the 0.6-V diode drop and the 0.2-V current
analogue signal developed across R2. Consequently, with 100 turns on the secondary and
a single current-linked turn on the primary, the voltage drop on the primary winding will
only be 8 mV. The primary insertion loss is thus very small.
For forward converter applications, the peak value of the primary current at the end
of an “on” period is the required current-limiting value (this indicates the peak current
flowing in the output inductor). Hence the secondary current must reflect a rising current
during the “on” period, and this sets a limit on the magnetizing current. A pulse-by-pulse
voltage analogue of the current is given by the voltage across R2. Hence, a fast-acting cur-
rent limit may be introduced at this point in the control circuit, using the peak analogue
voltage information, or the current signal may be used in a current-mode control circuit.
Step 4, Check Magnetizing Current
The current to be measured is 10 A, and the core chosen as the initial selection is the TDK
#T6-12–3 (Table 2.15.1) in a H5B2 material. This material has a permeability of 7500 and
a low remanence value B
of 40 mT. The next step is to check that the magnetization current
is acceptably small.
In this example, a single-layer secondary winding of 100 turns of #34 AWG is to be
used on the secondary.
To calculate the magnetization current, the secondary inductance is required. If the A
value of the core is known, then the inductance will be given by
L N A
In this example the A
value is not known, and so the inductance will be calculated from
the basic formula:
4P r 10
permeability of core
N number of secondary turns
core factor (ratio of effective core area to effective magnetic path length, m)
14. CURRENT TRANSFORMERS
For the H5B2 material, the permeability is 7500 and the core factor is (1/30.2) r 10
so the inductance can be calculated:
The slope of the magnetization current, dI/dt, can be calculated from
e 0 .8 V
25 .8 A/s
At the end of the 10-Ms pulse, I
0.258 mA. This magnetization current transforms to
the primary as 25.8 mA, a negligible error in 10 A.
It has been shown that the secondary current will be a true transform of the primary cur-
rent with a loss of only ¼ mA in 100 mA. Although this indicates that a smaller core could
be used, it can be difficult to wind the secondary on a core much smaller than this.
The value of the burden resistor R2 required to develop the chosen signal voltage of 0.2 V
can be calculated:
In this example, I
, or approximately 100 mA, as referred to the secondary.
Although the primary current pulses are unidirectional with a large mean DC value, the core
will not saturate, as the flux density falls to the remanence value B
after each current pulse.
To permit this reset action, the voltage on both primary and secondary must reverse (fly
back) during the “off” period. This requires that the reverse currents be blocked except for
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