NCP1205DR2 ON Semiconductor, NCP1205DR2 Datasheet - Page 10

• Manufacturer Part Number: NCP1205DR2
• Description: IC CTRLR PWM CM OVP HV 16SOIC
• Manufacturer: ON Semiconductor

Specifications of NCP1205DR2

• Output Isolation: Isolated
• Frequency Range: 90 ~ 125kHz
• Voltage - Input: 8 ~ 30 V
• Operating Temperature: -25°C ~ 150°C
• Package / Case: 16-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: NCP1205DR2OS

In order to clarify the device behavior, we can distinguish the
following simplified operating phases:
Advantages of the Method
NCP1205 brings the following advantages:





By implementing the aforementioned control scheme, the
1. The load is at its nominal value. The SMPS operates in
2. The load starts to decrease and the free- -running
3. The frequency can no longer naturally increase
4. The peak current has reached the internal minimum
5. To further reduce the transmitted power (V
a first order system (at low frequencies) and thus
naturally eases the feedback loop compensation.
smooth turn- -off conditions.
brought by Coss and all the parasitic capacitances.
system into Pulse Duration Modulation. This method
prevents from generating uncontrolled output ripple as
with hysteretic controllers.
the frequency goes down, you ensure that this level is
low enough to avoid transformer acoustic noise
generation even at audible frequencies.
Discontinuous only operation: in DCM, the Flyback is
A low- -cost secondary rectifier can be used due to
Valley switching ensures minimum switching losses
By folding back the switching frequency, you turn the
By letting you control the peak current value at which
borderline conduction mode and the switching
frequency is imposed by the external elements (Vin,
Lp, Ip, Vout). The MOSFET is turned on at the
minimum drain- -source level.
frequency hits the internal clamp.
because of the clamp. The frequency is now controlled
by the internal VCO but remains constant. The peak
current finds no other option that diminishing to satisfy
equation (1).
ceiling level and is now frozen for the remaining
cycles.
the VCO decreases the switching frequency. In case of
output overshoot, the VCO could decrease the
frequency down to zero. When the overshoot has gone,
V
operation.
FB
diminishes again and the IC smoothly resumes its
FB
goes up),
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10
Detailed Description
the NCP1205.
Free- -Running Operation
is dictated by the external elements. We can split the different
switching sections in two separated instants. In the following
text we use the internal error voltage, Verr. This level is
elaborated in Figure 13. Verr is linked to VFB (pin 4) by the
following formula:
reach the peak current setpoint imposed by the level on FB
pin (pin 4). Since this level is internally divided by three, the
peak setpoint is simply:
the inductance value and the rectified DC input voltage by:
definition:
transformer depends on the reset voltage applied at the
switch opening. During the conduction time of the
secondary diode, the primary side of the transformer
undergoes a reflected voltage of: [Np/Ns . (Vf + Vout)]. This
voltage applied on the primary inductance dictates the time
needed to decrease from Ip down to zero:
frequency of the SMPS operating in Borderline Conduction
Mode (BCM):
ton + toff =
The following sections describe the internal behavior of
As previously said, the operating frequency at nominal load
ON time: The ON time is given by the time it takes to
The rising slope of the peak current is also dependent on
By combining both equations, we obtain the ON time
OFF time: The time taken by the demagnetization of the
By adding ton + toff, we obtain the natural switching
ton =
· Ip =
3 · Rsense
Verr · Lp
Vin DC
toff =
Ipk =

Lp
Np
Ns
Verr = 10 − 3 · V FB
· (Vout + Vf)
dIL
dt
· Ip =
3 · Rsense

Np
Ns
·
⎪ ⎪
⎪
⎪
=
Vin DC
· (Vout + Vf)
Lp · Verr
1
Vin DC
1
Vin DC · 3 · Rsense
Lp
Lp

Lp · V ERR
+
· 3 · Rsense
· Verr

Np
Ns

· (Vout + Vf)
1
(eq. 2)
(eq. 3)
(eq. 4)
(eq. 5)
(eq. 6)
(eq. 7)

⎤
⎪
⎦